This doesn't happen to me. I have one media item linked to multiple people, multiple facts, or multiple citations. FTM does not duplicate the media item, which I can verify by looking in my media folder.
There is an acknowledged bug, however, that does happen from time to time when you merge a record via FTM that has a citation media item and then save an adjacent record via Ancestry.com that uses the same citation media item. This is the only bug I've seen that actually duplicates anything. (This would only create one duplicate however, even if you saved everyone on the same 1940 census page.)
If you're getting multiple duplicates that can be seen in your media folder, then my guess is that it may be user error, not FTM. Here are two things to consider.
(1) If you're manually adding the image files for the 1940 census, it's possible that you're copying the file multiple times rather than linking the same media item to multiple people, facts, and citations.
(2) When you get citation media from merging census records in FTM or saving them on Ancestry.com, they're all generically named with worthless captions and file names. For example, you might have 50+ images that say 1940 US Federal Census-1, 1940 US Federal Census-2, 1940 US Federal Census-3, etc., but these are actually different images. (Known bug excepted.)
If these two points don't address your problem, then please describe what you're doing in FTM or at Ancestry.com that results in these files, and maybe we can figure out how to prevent it.
There is an acknowledged bug, however, that does happen from time to time when you merge a record via FTM that has a citation media item and then save an adjacent record via Ancestry.com that uses the same citation media item. This is the only bug I've seen that actually duplicates anything. (This would only create one duplicate however, even if you saved everyone on the same 1940 census page.)
If you're getting multiple duplicates that can be seen in your media folder, then my guess is that it may be user error, not FTM. Here are two things to consider.
(1) If you're manually adding the image files for the 1940 census, it's possible that you're copying the file multiple times rather than linking the same media item to multiple people, facts, and citations.
(2) When you get citation media from merging census records in FTM or saving them on Ancestry.com, they're all generically named with worthless captions and file names. For example, you might have 50+ images that say 1940 US Federal Census-1, 1940 US Federal Census-2, 1940 US Federal Census-3, etc., but these are actually different images. (Known bug excepted.)
If these two points don't address your problem, then please describe what you're doing in FTM or at Ancestry.com that results in these files, and maybe we can figure out how to prevent it.