>Normally, when a break happens, there's another copy of the DNA sequence in the cell - remember you have two copies of each chromosome: one from your mom and one from your dad. So the repair mechanism looks for another similar sequence and copies it (oversimplifying here) to patch the hole.

HDR is mainly active in S and G2 phase, where you get up to four copies of each chromosome - two maternal, two paternal. That provides additional templates for repair (or let's say a stalled replication fork ripped both paternal sister chromatids apart - you then still have two maternal templates available).