Looks like it was being exploited on Bytecoin

[![enter image description here][1]][1]


  [1]: https://i.sstatic.net/bgGru.jpg

For example, these 2 transactions spend the [output](http://chainradar.com/bcn/transaction/07a09e3c26d8ffc2e890713a69974e943a23ef6ad65b3bcbfc2b0f0da1add8f4) twice!

http://chainradar.com/bcn/transaction/cef289d7fab6e35ac123db8a3f06f7675b48067e0dff185c72b140845b8b3b23

http://chainradar.com/bcn/transaction/7e418cc77935cc349f007cd5409d2b6908e4130321fa6f97ee0fee64b000ff85

Notice how the key image differs only in last 2 digits! That's because it's forged - ie 2 key images for 1 output, allowing you to spend it twice.

Note that it's [later patched](https://www.reddit.com/r/Monero/comments/6buu5j/disclosure_of_a_major_bug_in_cryptonotebased/dhrclct/) in the code, but the patch applies only after [Bytecoin block 1267000](https://github.com/amjuarez/bytecoin/commit/abca08734ce2518257fb8beb3b48431e46e59c8e#diff-17bedea6713f803f8a6f14075458481f), so those double-spend transactions are still in the Bytecoin chain and have artificially inflated the supply.