diff --git a/modules/dependencies.nix b/modules/dependencies.nix
index 32302a55..e5de3ddf 100644
--- a/modules/dependencies.nix
+++ b/modules/dependencies.nix
@@ -28,7 +28,15 @@ let
       };
   };
 
-  attrPathType = with types; coercedTo str lib.toList (listOf str);
+  # Motivation:
+  # If one were to define `__depPackages.foo.default = "gzip";` in two places (by accident),
+  # the module system would merge the two definitions as `["gzip" "gzip"]`.
+  #
+  # Solution:
+  # -> Make attrPathType unique so the option can only be set once.
+  attrPathType =
+    with types;
+    unique { message = "attrPathType must be unique"; } (coercedTo str lib.toList (listOf str));
 
   literalExpressionType = lib.types.mkOptionType {
     name = "literal-expression";