Using msg.value inside a loop is dangerous because this might allow the sender to “re-use” the msg.value multiple times, while only providing it once:
When a transaction calls a function in a smart contract and includes a transfer of ether (i.e., msg.value), that ether is made available to the contract upon execution. If msg.value is used inside a loop, the same ether value might be referenced multiple times, effectively allowing the sender to "re-use" the ether value.
Here's why this happens:
Each time the loop iterates and encounters msg.value, it uses the ether sent with the transaction. Multiple iterations may each reference the same msg.value, leading to unintended consequences. This allows the sender to effectively "re-use" the same ether value in each iteration of the loop.
