diff --git a/lib/options.nix b/lib/options.nix
index f5012848b05a..0d1d90efe217 100644
--- a/lib/options.nix
+++ b/lib/options.nix
@@ -254,13 +254,31 @@ rec {
     else if all isInt list && all (x: x == head list) list then head list
     else throw "Cannot merge definitions of `${showOption loc}'. Definition values:${showDefs defs}";
 
+  /*
+    Require a single definition.
+
+    WARNING: Does not perform nested checks, as this does not run the merge function!
+    */
   mergeOneOption = mergeUniqueOption { message = ""; };
 
-  mergeUniqueOption = { message }: loc: defs:
-    if length defs == 1
-    then (head defs).value
-    else assert length defs > 1;
-      throw "The option `${showOption loc}' is defined multiple times while it's expected to be unique.\n${message}\nDefinition values:${showDefs defs}\n${prioritySuggestion}";
+  /*
+    Require a single definition.
+
+    NOTE: When the type is not checked completely by check, pass a merge function for further checking (of sub-attributes, etc).
+   */
+  mergeUniqueOption = args@{
+      message,
+      # WARNING: the default merge function assumes that the definition is a valid (option) value. You MUST pass a merge function if the return value needs to be
+      #   - type checked beyond what .check does (which should be very litte; only on the value head; not attribute values, etc)
+      #   - if you want attribute values to be checked, or list items
+      #   - if you want coercedTo-like behavior to work
+      merge ? loc: defs: (head defs).value }:
+    loc: defs:
+      if length defs == 1
+      then merge loc defs
+      else
+        assert length defs > 1;
+        throw "The option `${showOption loc}' is defined multiple times while it's expected to be unique.\n${message}\nDefinition values:${showDefs defs}\n${prioritySuggestion}";
 
   /* "Merge" option definitions by checking that they all have the same value. */
   mergeEqualOption = loc: defs:
diff --git a/lib/tests/modules.sh b/lib/tests/modules.sh
index 072b92b38365..b3bbdf9485ac 100755
--- a/lib/tests/modules.sh
+++ b/lib/tests/modules.sh
@@ -407,6 +407,16 @@ checkConfigOutput "{}" config.submodule.a ./emptyValues.nix
 checkConfigError 'The option .int.a. is used but not defined' config.int.a ./emptyValues.nix
 checkConfigError 'The option .nonEmptyList.a. is used but not defined' config.nonEmptyList.a ./emptyValues.nix
 
+# types.unique
+#   requires a single definition
+checkConfigError 'The option .examples\.merged. is defined multiple times while it.s expected to be unique' config.examples.merged.a ./types-unique.nix
+#   user message is printed
+checkConfigError 'We require a single definition, because seeing the whole value at once helps us maintain critical invariants of our system.' config.examples.merged.a ./types-unique.nix
+#   let the inner merge function check the values (on demand)
+checkConfigError 'A definition for option .examples\.badLazyType\.a. is not of type .string.' config.examples.badLazyType.a ./types-unique.nix
+#   overriding still works (unlike option uniqueness)
+checkConfigOutput '^"bee"$' config.examples.override.b ./types-unique.nix
+
 ## types.raw
 checkConfigOutput '^true$' config.unprocessedNestingEvaluates.success ./raw.nix
 checkConfigOutput "10" config.processedToplevel ./raw.nix
diff --git a/lib/tests/modules/types-unique.nix b/lib/tests/modules/types-unique.nix
new file mode 100644
index 000000000000..115be0126975
--- /dev/null
+++ b/lib/tests/modules/types-unique.nix
@@ -0,0 +1,27 @@
+{ lib, ... }:
+let
+  inherit (lib) mkOption types;
+in
+{
+  options.examples = mkOption {
+    type = types.lazyAttrsOf
+      (types.unique
+        { message = "We require a single definition, because seeing the whole value at once helps us maintain critical invariants of our system."; }
+        (types.attrsOf types.str));
+  };
+  imports = [
+    { examples.merged = { b = "bee"; }; }
+    { examples.override = lib.mkForce { b = "bee"; }; }
+  ];
+  config.examples = {
+    merged = {
+      a = "aye";
+    };
+    override = {
+      a = "aye";
+    };
+    badLazyType = {
+      a = true;
+    };
+  };
+}
diff --git a/lib/types.nix b/lib/types.nix
index 7b2062f13059..12bf18633e3a 100644
--- a/lib/types.nix
+++ b/lib/types.nix
@@ -614,23 +614,12 @@ rec {
       nestedTypes.elemType = elemType;
     };
 
-    # Value of given type but with no merging (i.e. `uniq list`s are not concatenated).
-    uniq = elemType: mkOptionType rec {
-      name = "uniq";
-      inherit (elemType) description descriptionClass check;
-      merge = mergeOneOption;
-      emptyValue = elemType.emptyValue;
-      getSubOptions = elemType.getSubOptions;
-      getSubModules = elemType.getSubModules;
-      substSubModules = m: uniq (elemType.substSubModules m);
-      functor = (defaultFunctor name) // { wrapped = elemType; };
-      nestedTypes.elemType = elemType;
-    };
+    uniq = unique { message = ""; };
 
     unique = { message }: type: mkOptionType rec {
       name = "unique";
       inherit (type) description descriptionClass check;
-      merge = mergeUniqueOption { inherit message; };
+      merge = mergeUniqueOption { inherit message; inherit (type) merge; };
       emptyValue = type.emptyValue;
       getSubOptions = type.getSubOptions;
       getSubModules = type.getSubModules;
diff --git a/nixos/doc/manual/development/option-types.section.md b/nixos/doc/manual/development/option-types.section.md
index f9c7ac80018e..04edf99e70b0 100644
--- a/nixos/doc/manual/development/option-types.section.md
+++ b/nixos/doc/manual/development/option-types.section.md
@@ -326,7 +326,7 @@ Composed types are types that take a type as parameter. `listOf
 `types.uniq` *`t`*
 
 :   Ensures that type *`t`* cannot be merged. It is used to ensure option
-    definitions are declared only once.
+    definitions are provided only once.
 
 `types.unique` `{ message = m }` *`t`*