Duplicated hash values in mining reward This transaction is contained in two MainChain-Blocks:

https://blockchain.info/tx/e3bf3d07d4b0375638d5f1db5255fe07ba2c4cb067cd81b84ee974b6585fb468

On Blockexplorer there is a note of this anomaly:

https://blockexplorer.com/block/00000000000743f190a18c5577a3c2d2a1f610ae9601ac046a38084ccb7cd721

As I understand, those 50 BTC are lost because only one of the rewards can be redeemed. 

If so there will never be 21 million Bitcoin available, because those, or more of this sort, are missing?
This is a known bug in the early days of bitcoin.

Was fixed by BIP-30 https://github.com/bitcoin/bips/blob/master/bip-0030.mediawiki

There is a discussion here: https://bitcointalk.org/index.php?topic=216938

And yes, 50+50 bitcoins are missed forever.

In fact, there are much more missed bitcoins. For example, 2500 bitcoins were gone to nowhere 
in block 0000000000000449ee5b94ba7a051caffff5c23d6a03335f6e20e3985b5ffa61