Basically after an hard fork, the distributed ledger gets copied and if there is no replay protection on the second ledger, the copy, every transaction made in either of the chain can be replayed in the other.
The first ledger also has no replay protection and to implement it another hard fork would be necessary, which could be possible but not in 3 months time.
So, how to avoid it?
I tried to keep it short, if you'd like more info this 3 min video explains it well:
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