ご回答ありがとうございます。
アドバイス頂いた点について実施してみました。
結果としては以下の通りとなり、ご推察頂いた状態にはなっていないようです。
１．「BBFILE_COLLECTIONS += "openembedded-layer"」はなし
２．「BBFILE_COLLECTIONS += "openembedded-layer"」は一か所
３．「BBFILE_COLLECTIONS += "openembedded-layer"」はなし

当方、他のレイヤの組み込みも行っている為、何を行った後に発生するようになるのかを確認してみようと思います。

以上よろしくお願いいたします。

以下は実施結果です。

確認【1. build/conf/bblayers.confに「BBFILE_COLLECTIONS += "openembedded-layer"」が存在する】
=====================================================
$ cat build/conf/bblayers.conf
# POKY_BBLAYERS_CONF_VERSION is increased each time build/conf/bblayers.conf
# changes incompatibly
POKY_BBLAYERS_CONF_VERSION = "2"

BBPATH = "${TOPDIR}"
BBFILES ?= ""

BBLAYERS ?= " \
${TOPDIR}/../meta-gplv2 \
${TOPDIR}/../poky/meta \
${TOPDIR}/../poky/meta-poky \
${TOPDIR}/../poky/meta-yocto-bsp \
${TOPDIR}/../meta-renesas/meta-rz-common \
${TOPDIR}/../meta-renesas/meta-rzg2l \
${TOPDIR}/../meta-openembedded/meta-oe \
${TOPDIR}/../meta-openembedded/meta-python \
${TOPDIR}/../meta-openembedded/meta-multimedia \
/home/takahiro/work/meta-qt5 \
/home/takahiro/work/meta-rz-features/meta-rz-graphics \
/home/takahiro/work/meta-rz-features/meta-rz-codecs \
/home/takahiro/work/meta-openembedded/meta-filesystems \
/home/takahiro/work/meta-openembedded/meta-networking \
/home/takahiro/work/meta-virtualization \
"
=====================================================

確認【2. meta-oe/conf/layers.confに「BBFILE_COLLECTIONS += "openembedded-layer"」が2つ以上存在する】
=====================================================
$ cat meta-openembedded/meta-oe/conf/layer.conf
# It really depends on order of the layers appearing in BBLAYERS
# variable in toplevel bblayers.conf file, where bitbake will search
# for .inc files and others where bitbake uses BBPATH since it will
# search the directories from first to last as specified in BBPATH
# Therefore if you want a given layer to be considered high priority
# for the .inc and .conf etc. then consider it adding at the beginning
# of BBPATH. For bblayers bitbake will use BBFILES_PRIORITY to resolve
# the recipe contention so the order of directories in BBFILES does
# not matter.

# We have a conf and classes directory, append to BBPATH
BBPATH .= ":${LAYERDIR}"

# We have a recipes directory, add to BBFILES
BBFILES += "${LAYERDIR}/recipes-*/*/*.bb ${LAYERDIR}/recipes-*/*/*.bbappend"

BBFILE_COLLECTIONS += "openembedded-layer"
BBFILE_PATTERN_openembedded-layer := "^${LAYERDIR}/"

# Define the priority for recipes (.bb files) from this layer,
# choosing carefully how this layer interacts with all of the
# other layers.

BBFILE_PRIORITY_openembedded-layer = "6"

# only activates content when identified layers are present,
# to ensure yocto compatibility check pass
BBFILES_DYNAMIC += " \
meta-python:${LAYERDIR}/dynamic-layers/meta-python/recipes-*/*/*.bb \
perl-layer:${LAYERDIR}/dynamic-layers/perl-layer/recipes-*/*/*.bb \
"

# This should only be incremented on significant changes that will
# cause compatibility issues with other layers
LAYERVERSION_openembedded-layer = "1"

LAYERDEPENDS_openembedded-layer = "core"

LAYERSERIES_COMPAT_openembedded-layer = "thud warrior zeus dunfell"

LICENSE_PATH += "${LAYERDIR}/licenses"

PREFERRED_RPROVIDER_libdevmapper = "lvm2"

SIGGEN_EXCLUDERECIPES_ABISAFE += " \
mplayer-common \
fbset-modes \
gpsd-machine-conf \
distro-feed-configs \
ca-certificates \
pointercal \
"

SIGGEN_EXCLUDE_SAFE_RECIPE_DEPS += " \
android-tools->android-tools-conf \
usb-modeswitch-data->usb-modeswitch \
lmsensors->lmsensors-config \
phoronix-test-suite->bash \
phoronix-test-suite->python3 \
phoronix-test-suite->php \
phoronix-test-suite->lsb-release \
phoronix-test-suite->util-linux \
phoronix-test-suite->busybox \
phoronix-test-suite->shared-mime-info \
phoronix-test-suite->desktop-file-utils \
"

# ttf.inc inherits fontcache which adds fontconfig-utils to RDEPENDS
SIGGEN_EXCLUDE_SAFE_RECIPE_DEPS += " \
terminus-font->fontconfig \
ttf-abyssinica->fontconfig \
ttf-arphic-uming->fontconfig \
ttf-dejavu->fontconfig \
ttf-droid->fontconfig \
ttf-gentium->fontconfig \
ttf-hunkyfonts->fontconfig \
ttf-inconsolata->fontconfig \
ttf-liberation->fontconfig \
ttf-liberation-sans-narrow->fontconfig \
ttf-lklug->fontconfig \
ttf-lohit->fontconfig \
ttf-noto-emoji->fontconfig \
ttf-pt-sans->fontconfig \
ttf-roboto->fontconfig \
ttf-mplus->fontconfig \
ttf-sazanami->fontconfig \
ttf-tlwg->fontconfig \
ttf-ubuntu-font-family->fontconfig \
ttf-vlgothic->fontconfig \
ttf-wqy-zenhei->fontconfig \
source-han-sans-cn-fonts->fontconfig \
source-han-sans-jp-fonts->fontconfig \
source-han-sans-kr-fonts->fontconfig \
source-han-sans-tw-fonts->fontconfig \
"

DEFAULT_TEST_SUITES_pn-meta-oe-ptest-image = " ${PTESTTESTSUITE}"
=====================================================

確認【3. bitbake-layers show-layersで表示されるレイヤのlayers.confに「BBFILE_COLLECTIONS += "openembedded-layer"」が存在する】
=====================================================
$ bitbake-layers show-layers
NOTE: Starting bitbake server...
layer path priority
==========================================================================
meta-gplv2 /home/takahiro/work/build/../meta-gplv2 1
meta /home/takahiro/work/build/../poky/meta 5
meta-poky /home/takahiro/work/build/../poky/meta-poky 5
meta-yocto-bsp /home/takahiro/work/build/../poky/meta-yocto-bsp 5
meta-rz-common /home/takahiro/work/build/../meta-renesas/meta-rz-common 15
meta-rzg2l /home/takahiro/work/build/../meta-renesas/meta-rzg2l 15
meta-oe /home/takahiro/work/build/../meta-openembedded/meta-oe 6
meta-python /home/takahiro/work/build/../meta-openembedded/meta-python 7
meta-multimedia /home/takahiro/work/build/../meta-openembedded/meta-multimedia 6
meta-qt5 /home/takahiro/work/meta-qt5 7
meta-rz-graphics /home/takahiro/work/meta-rz-features/meta-rz-graphics 16
meta-rz-codecs /home/takahiro/work/meta-rz-features/meta-rz-codecs 16
meta-filesystems /home/takahiro/work/meta-openembedded/meta-filesystems 6
meta-networking /home/takahiro/work/meta-openembedded/meta-networking 5
meta-virtualization /home/takahiro/work/meta-virtualization 8
=====================================================

みつきん様、ご回答ありがとうございます。

今回使用している環境は、「RZ/G Verified Linux Package Version 3.0.3（DISTRO_VERSION = "3.1.21"）」ですので、
新規で起こしたそのままの環境にて、ビルド初期化スクリプトだけ走らせてみました。
結果としては、みつきん様のご指摘通り、この時点でレイヤ「meta-oe」が含まれている環境であることがわかりました。

今回、レイヤ「meta-oe」を追加しようとした経緯は「hiredis」を追加する為でした。（https://yoctobbq.lineo.co.jp/?q=node/496）
その時点でレイヤ「meta-oe」含まれているか確認せずに「bitbake-layers add-layer」を行う様に自作スクリプトを変更した結果でした。

今回の経験で現在のレイヤ構成を確認する方法を知ることが出来ました。

以上、大変お手数をお掛けしました。ありがとうございました。本件クローズとなります。

以下、確認した結果です。

ビルド初期化スクリプトを実行します。
==============================================================================
$TEMPLATECONF=$PWD/meta-renesas/meta-rzg2l/docs/template/conf/ source ./poky/oe-init-build-env build

Start initializing the build environment ...
You had no conf/local.conf file. This configuration file has therefore been
created for you with some default values. You may wish to edit it to, for
example, select a different MACHINE (target hardware). See conf/local.conf
for more information as common configuration options are commented.

You had no conf/bblayers.conf file. This configuration file has therefore been
created for you with some default values. To add additional metadata layers
into your configuration please add entries to conf/bblayers.conf.

The Yocto Project has extensive documentation about OE including a reference
manual which can be found at:
https://docs.yoctoproject.org

For more information about OpenEmbedded see their website:
https://www.openembedded.org/

### Shell environment set up for builds. ###

You can now run 'bitbake <target>'

Targets are:
core-image-minimal
core-image-bsp
core-image-weston
core-image-qt
==============================================================================

レイヤ構成を確認します。既に「mea-oe」が含まれています。
==============================================================================
$ bitbake-layers show-layers
NOTE: Starting bitbake server...
layer path priority
==========================================================================
meta-gplv2 /home/takahiro/work/build/../meta-gplv2 1
meta /home/takahiro/work/build/../poky/meta 5
meta-poky /home/takahiro/work/build/../poky/meta-poky 5
meta-yocto-bsp /home/takahiro/work/build/../poky/meta-yocto-bsp 5
meta-rz-common /home/takahiro/work/build/../meta-renesas/meta-rz-common 15
meta-rzg2l /home/takahiro/work/build/../meta-renesas/meta-rzg2l 15
meta-oe /home/takahiro/work/build/../meta-openembedded/meta-oe 6 ★ココ★
meta-python /home/takahiro/work/build/../meta-openembedded/meta-python 7
meta-multimedia /home/takahiro/work/build/../meta-openembedded/meta-multimedia 6
==============================================================================