[Avodah] Sameach Tesamach vs Samach Tesamach vs Sameach Tesemeach

[I. Balbin]

There is nothing like one's own child's impending wedding to re- 
acquaint oneself with the various issues involved. I certainly know a  
lot more about the process than I did during our wedding many moons ago.

In Sheva Brochos, the "standard" text reads Sameach Tesamach Reim ...

Chabad are makpid to say "Samach Tesamach"

I listened to the Rov, RYBS zt"l say the brocha and he said "Sameach  
Tesameach"

Alas, I don't have much time left to research this, but I'd be  
surprised if someone hadn't already done so.

Can anyone provide pointers or further elucidation of this text?