Type 4712 indicates this is an in-game produced unit that causes the problem.
Just out of curiosity, could you open the save game file with notepad or wordpad, and then search for 456570
When you find it, it should be on a line with something like this:
type = 4712 id = 456570
Now, this portion will be part of a larger structure (using { and } characters). Are, within the larger structure, the following present?
type = paratrooper model = n (where n is a number between 0 and 3)
If so, use the search function of notepad/wordpad to find the next occurence of 456570. It should find it, as the game engine complains about a duplicate number. Now, examine the second occurence. Does it also specify:
type = paratrooper model = n (where n is a number between 0 and 3)
If so, are the other values you find (name, location, leader, etc) identical between the two occurences? If so, could you report back here.