Why the blocks in another chain were removed?
2017-May-20 10: 32: 47.581300 WARNING Block f5abab4438ad925817511800b3e23ff14c48284c8a847fad0f93d639b61d7b0f added to alternative chain. Index: 320
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2017-May-20 10: 35: 46.825557 WARNING Block 724dba0307174b5a73bb756b5215000c0f29dd01299b21423ef723861790cc81 added to alternative chain. Index: 373
Same chain, it's just indexed into a rocks DB. This is message forknoted prints when cryptonote miner votes comes into play as outlined in the cryptonote whitepaper.
Also prented if a block orphans and is replace with an alternate block height.
The other message regarding spent image key is refering to ring signature hiding sender and receiver address and creating a random view key but the one generated was already used so the system prints that message until the DB generate a key that has not been used to prevent tracable transactions.