Help: Clarify ordering of list(REMOVE_DUPLICATES)

This also adds a test to enforce the order behavior of
list(REMOVE_DUPLICATES).

Help/command/list.rst

@@ -196,7 +196,8 @@ Removes items at given indices from the list.
 
   list(REMOVE_DUPLICATES <list>)
 
-Removes duplicated items in the list.
+Removes duplicated items in the list. The relative order of items is preserved,
+but if duplicates are encountered, only the first instance is preserved.
 
 .. _TRANSFORM:
 

Tests/RunCMake/list/REMOVE_DUPLICATES-PreserveOrder.cmake

@@ -0,0 +1,5 @@
+set(mylist "b;c;b;a;a;c;b;a;c;b")
+list(REMOVE_DUPLICATES mylist)
+if(NOT mylist STREQUAL "b;c;a")
+  message(SEND_ERROR "Expected b;c;a, got ${mylist}")
+endif()

Tests/RunCMake/list/RunCMakeTest.cmake

@@ -24,6 +24,8 @@ run_cmake(SUBLIST-TooManyArguments)
 
 run_cmake(REMOVE_AT-EmptyList)
 
+run_cmake(REMOVE_DUPLICATES-PreserveOrder)
+
 run_cmake(FILTER-NotList)
 run_cmake(REMOVE_AT-NotList)
 run_cmake(REMOVE_DUPLICATES-NotList)