diff --git a/pkgs/overlays/unstable-from-source.nix b/pkgs/overlays/unstable-from-source.nix
index cd2ac95..4b40bcb 100644
--- a/pkgs/overlays/unstable-from-source.nix
+++ b/pkgs/overlays/unstable-from-source.nix
@@ -17,6 +17,13 @@ let
     "parsed"
   ];
 
+  # if overlay is found in previous layer, then it was provided in "overlays" argument
+  # otherwise, it is provided in "crossOverlays" argument.
+  overlays = self.lib.groupBy (overlay:
+    if self.lib.elem overlay self.buildPackages.overlays then "allLayers"
+    else "finalLayer"
+  ) self.overlays;
+
   unstablePkgs = import unstableSource {
     # localSystem -> pkgs.stdenv.buildPlatform
     localSystem = sanitizePlatform self.stdenv.buildPlatform;
@@ -24,11 +31,10 @@ let
     # passing below
     # config -> pkgs.config
     config = self.config;
-    # overlays -> partial of pkgs.overlays
-    overlays = self.overlays;
-    # crossOverlays -> partial of pkgs.overlays
-    # crossOverlays are merged to overlays, not sure what issues that might raise.
-    # ignoring.
+    # overlays -> pkgs.buildPackages.overlays
+    overlays = overlays.allLayers;
+    # crossOverlays -> pkgs.overlays without pkgs.buildPackages.overlays
+    crossOverlays = overlays.finalLayer;
   } // self.lib.optionalAttrs (
     self.lib.systems.equals self.stdenv.buildPlatform self.stdenv.hostPlatform
   ) {