How will you identify who has sent which transaction?!
There could be cheaters trying to exploit this by sending somebody else transaction hashes.

I mean is easy to find out in the block explorer the transactions that were sent out of exchnages to DIMCOIN address
and try to claim those.

Example:
User A sent from exchange to ICO.
User B has not sent anything.

User B checks the block explorer and find TX hash there.
User B tries claim the transaction that wasn't his and sends to you TX hash.

User A tries to claim his own transaction and send TX hash which is equal to TX hash that user B sent to you.

Neither of them have filled your form during the ICO.

How do you identify which one is laying and who sent the transaction?