abuild: return error if there are more than one maintainer

fixes #5421

abuild.in

@@ -286,6 +286,8 @@ default_sanitycheck() {
 	[ -n "$checkdepend" ] && spell_error checkdepend checkdepends
 
 	check_maintainer || die "Provide a valid RFC822 maintainer address"
+	[ $(grep '^# *Maintainer:' "$APKBUILD" | wc -l) -gt 1 ] \
+		&& die "More than one maintainer"
 	check_license || warning "Please use valid SPDX license identifiers found at: https://spdx.org/licenses"
 
 	check_depends_dev || warning "depends_dev found but no development subpackage found"