If you need to save more than once in a mission, then you need to delete the old save game file. It can be found in the directory UsersSavedCampaigns1985. ALT-TAB out of the game, make a backup copy (trust me!) and then delete the save file. ALT-TAB back and press ESC to pause the game. You should now be able to save the game again. You can do this as many times as you like.