diff --git a/scripts/libmakepkg/buildenv/debugflags.sh.in b/scripts/libmakepkg/buildenv/debugflags.sh.in
index 71022836..22a79281 100644
--- a/scripts/libmakepkg/buildenv/debugflags.sh.in
+++ b/scripts/libmakepkg/buildenv/debugflags.sh.in
@@ -25,14 +25,15 @@ LIBMAKEPKG_BUILDENV_DEBUGFLAGS_SH=1
 MAKEPKG_LIBRARY=${MAKEPKG_LIBRARY:-'@libmakepkgdir@'}
 
 source "$MAKEPKG_LIBRARY/util/option.sh"
+source "$MAKEPKG_LIBRARY/util/util.sh"
 
 buildenv_functions+=('buildenv_debugflags')
 
 buildenv_debugflags() {
 	if check_option "debug" "y" && ! check_option "buildflags" "n"; then
-		DEBUG_CFLAGS+=" -ffile-prefix-map=$srcdir=${DBGSRCDIR:-/usr/src/debug}/${pkgbase}"
-		DEBUG_CXXFLAGS+=" -ffile-prefix-map=$srcdir=${DBGSRCDIR:-/usr/src/debug}/${pkgbase}"
-		CFLAGS+=" $DEBUG_CFLAGS"
-		CXXFLAGS+=" $DEBUG_CXXFLAGS"
+		append_once DEBUG_CFLAGS "-ffile-prefix-map=$srcdir=${DBGSRCDIR:-/usr/src/debug}/${pkgbase}"
+		append_once DEBUG_CXXFLAGS "-ffile-prefix-map=$srcdir=${DBGSRCDIR:-/usr/src/debug}/${pkgbase}"
+		append_once CFLAGS "$DEBUG_CFLAGS"
+		append_once CXXFLAGS "$DEBUG_CXXFLAGS"
 	fi
 }
diff --git a/scripts/libmakepkg/buildenv/lto.sh.in b/scripts/libmakepkg/buildenv/lto.sh.in
index ebaa3656..290ad925 100644
--- a/scripts/libmakepkg/buildenv/lto.sh.in
+++ b/scripts/libmakepkg/buildenv/lto.sh.in
@@ -25,14 +25,15 @@ LIBMAKEPKG_BUILDENV_LTO_SH=1
 MAKEPKG_LIBRARY=${MAKEPKG_LIBRARY:-'@libmakepkgdir@'}
 
 source "$MAKEPKG_LIBRARY/util/option.sh"
+source "$MAKEPKG_LIBRARY/util/util.sh"
 
 build_options+=('lto')
 buildenv_functions+=('buildenv_lto')
 
 buildenv_lto() {
 	if check_option "lto" "y" && ! check_option "buildflags" "n"; then
-		CFLAGS+=" ${LTOFLAGS:--flto}"
-		CXXFLAGS+=" ${LTOFLAGS:--flto}"
-		LDFLAGS+=" ${LTOFLAGS:--flto}"
+		append_once CFLAGS "${LTOFLAGS:--flto}"
+		append_once CXXFLAGS "${LTOFLAGS:--flto}"
+		append_once LDFLAGS "${LTOFLAGS:--flto}"
 	fi
 }
diff --git a/scripts/libmakepkg/buildenv/rust.sh.in b/scripts/libmakepkg/buildenv/rust.sh.in
index 6d2d378d..94d68f87 100644
--- a/scripts/libmakepkg/buildenv/rust.sh.in
+++ b/scripts/libmakepkg/buildenv/rust.sh.in
@@ -24,13 +24,14 @@ LIBMAKEPKG_BUILDENV_RUST_SH=1
 MAKEPKG_LIBRARY=${MAKEPKG_LIBRARY:-'@libmakepkgdir@'}
 
 source "$MAKEPKG_LIBRARY/util/option.sh"
+source "$MAKEPKG_LIBRARY/util/util.sh"
 
 buildenv_var+=('RUSTFLAGS' 'DEBUG_RUSTFLAGS')
 buildenv_functions+=('buildenv_rust')
 
 buildenv_rust() {
 	if check_option "debug" "y" && ! check_option "buildflags" "n"; then
-		DEBUG_RUSTFLAGS+=" --remap-path-prefix=$srcdir=${DBGSRCDIR:-/usr/src/debug}/${pkgbase}"
-		RUSTFLAGS+=" $DEBUG_RUSTFLAGS"
+		append_once DEBUG_RUSTFLAGS "--remap-path-prefix=$srcdir=${DBGSRCDIR:-/usr/src/debug}/${pkgbase}"
+		append_once RUSTFLAGS "$DEBUG_RUSTFLAGS"
 	fi
 }
\ No newline at end of file
diff --git a/scripts/libmakepkg/util/util.sh.in b/scripts/libmakepkg/util/util.sh.in
index bf3a83e2..83b8e57d 100644
--- a/scripts/libmakepkg/util/util.sh.in
+++ b/scripts/libmakepkg/util/util.sh.in
@@ -112,3 +112,13 @@ source_safe() {
 
 	eval "$shellopts"
 }
+
+# Append a string to a variable if the value is not already contained.
+# usage : append_once( $ref, $value )
+append_once() {
+	local -n var=$1
+	local value=$2
+	if [[ ! $var =~ (^| )"$value"($| ) ]]; then
+		var+=" $value"
+	fi
+}