No two letters selections on Countdown have ever been exactly identical (same letters in the same order). Nine selections match nine other selections in their first eight letters, though - for example, round 2 of episode 2085 and round 12 of episode 3853.
If we just look at R1 selections and find the longest matching prefix between two such selections, we find that episodes 1055 and 3721 were identical in their first seven letters, as were episodes 3948 and 4827.
If we disregard the order of the letters within a selection, there are loads of episodes with the same R1 selection as some other episode (I make it 444, if you count each occurrence of the same R1 selection separately).
There is only one instance of two episodes having the same R1 selection and the same R2 selection. These were episodes 153 and 154, so it was probably a shuffling failure rather than a remarkable coincidence.