I've seen many transactions with the same hash. However, is it safe to say that ledger/hash pairs are unique and there are no scenarios where two transactions on the same ledger share a hash?
I've seen many transactions with the same hash.
If transaction has the same hash as the other one it means they are identical.
You can have two transactions that are doing the same thing (ex. sending 1 XLM from A to B) but they need to have a different sequence numbers and this will make their hashes different.