When one blockchain forks into two blockchains, the result is an equal quantity of coins existing on both chains simultaneously. However, since both blockchains originated from the same parent chain, the networks of both chains will accept broadcasted transactions coming from either network.
As a result, it is possible that you intend to send just ETH or just ETC but accidentally send both simultaneously when broadcasting your transaction to the network. It is also possible for a malicious actor to rebroadcast your transaction to the other blockchain network in an attempt to undermine that networks credibility.
To read more about how replay attacks work, you can find more information here: https://themerkle.com/what-is-a-bitcoin-replay-attack/