CMake rule to copy one of multiple identically-named files - cmake

Question

Let's say we have a project like this:
foo
+-bar
+ +-header.h <-- copy if updated
+-baz
+ +-header.h <-- copy if updated
+-wibble
+ +-ni
+ +-header.h <-- copy if updated
target
+-header.h <-- to here
We have three possible candidates for header.h, but one final location; we're not using symbolic links for reasons. Note that there are actually around 350 header files with just a few duplicates, so listing each individually is troublesome, especially if these subdirectories update their header files. Currently, we do this unconditional copy:
file(glob_recurse HEADERS foo/*.h)
file(COPY ${HEADERS} DESTINATION target)
However, this rule only runs when we do a CMake rebuild and not when only header files change, so I would like to get this working for header file changes alone.
Now, looking at this answer I try something like this:
file(glob_recurse HEADERS foo/*.h)
foreach(file_i ${HEADERS})
get_filename_component(barename, ${file_i} NAME)
add_custom_command(
OUTPUT target/${barename}
COMMAND ${CMAKE_COMMAND}
ARGS -E copy ${file_i} target
)
endforeach( file_i )
However, CMake complains with the error:
CMake Error: Attempt to add a custom rule to output "target/header.h" which already has a custom rule.
I understand why I get the error, but I cannot think of any way to do what I want to do.