7/11
So when the same outputs are used, the coinbase transactions have the same transaction ID.
They obviously are 2 different transactions in 2 different blocks, and are paying 2 different things, but they both pay to the same output.
(Here the output is a pay-to-pub key)
8/11
This problem 1st was addressed in BIP30.
Blocks are not allowed to contain a TX with the same ID as a previously not-spent TX.
While this solved some of the problems, it's still not enough.
Also, the BIP has specific exceptions for the above-mentioned TXs.
9/11
Later in BIP34 a new rule was added.
Each coinbase TX is to include the height of the block.
As each block height is different, this guarantees each coinbase TX ID will be unique.
Blocks with such coinbase TXs are version2
Last version1 block was 227,835 was mined in 2013
10/11
This was a problem because it was not the desired behavior for Bitcoin TXs.
In some cases could have led to network splits.
Plus, could allow for some attacks, where you can reduce the confirmation of a TX to 1, making it more susceptible to double-spending.
(could****)
11/11
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