I was playing on a US East server today during lunch (12:40 EST or so) on Underground as Defenders.
There was a C4 plant at the first objective, and I was playing Bushwhacker while another teammate of mine was defusing as Skyhammer. He was nearly 1/3rd of the way through the defuse when I took over the defuse.
Since certain Mercs have faster defuse / repair rates (Fletcher, Bushwhacker, Proxy), they effectively “take over” the repair or defuse from any other merc. However in this instance, when the defuse was completed, the game gave overall credit to the Skyhammer teammate. e.g. “Skyhammer has defused the C4!”
We already have a problem where a lot of players don’t seem to realize that you CANNOT stack repair / defuses… the last thing I want to see is global credit given to them 
But in all seriousness, there should only ever be 1 ‘active’ player on a defuse / repair… so I’m not sure how the game still saw they contributed the last tick.