Hmm... I dunno... I was facing this dilemna last year. I had my first couple letter writers give me 3 versions each for the same LoR... one for Rads, one for Gas, and one "generic" LoR. But then later I found this out...
(taken from
http://www.ecfmg.org/eras/)
How many LoRs am I allowed to send?
You can send as many LoRs as you think you want to use. However, even if you send more than four, you can assign only four LoRs to any one program.
Most important: Each letter writer should write only one LoR for you. We attach LoRs strictly by name. If you send us ten LoRs and list ten names in MyERAS, we will attach Dr. Patel's LoR to his name, Dr. Ahmed's LoR to his name, etc.
If you list Dr. Smith twice, let's say as LoRs #3 and #4, we do not know which of the two letters that we have from Dr. Smith to attach to Dr. Smith #3 and which one to Dr. Smith #4.
Our high volume does not permit us to read your LoRs to determine for which specialty each is written. Furthermore, we do not know which LoR you have assigned to which specialty. So, in this example there is a 50 percent chance of mixing up the LoRs.
I don't know if they enforce this though...