Why is it said that if we have a duplicate ciphertext block it can leak our information?
The exact information leaked depends on the mode of operation that is in use.
The simplest case is ECB, where a duplicate ciphertext block means that the corresponding plaintext blocks are also equal.

For the CBC mode, when two ciphertext block are equal, i.e. $C_i=C_j$, we know that there was equality before applying the block cipher. This in turn implies:$$C_{i-1}\oplus P_i=C_{j-1}\oplus P_j.$$
Equivalently, you are learning that $P_{i}\oplus P_j=C_{i-1}\oplus C_{j-1}$.

EDIT The equality comes from the fact that in CBC mode, $C_i=E_K(C_{i-1}\oplus P_i)$ where $E_K$ denotes the block cipher in use.