[Ethereum] an uncle/ommer block

Uncles occur when a valid block does not make it into the long-term consensus. These orphaned blocks can be caused by high network latency, so I would say that a dramatic increase in the uncle rate indicates that the block gas limit is too high, thus making the blocks too large to propagate efficiently. Alternatively, it could indicate some other slow down of block propagation, due to DDOS attacks or some other network interference.

Yes, it is possible because the miner's won't necessarily know the other has found a solution. The news of these discoveries spreads via gossip, so there is always latency.

A very short summary of the way this is resolved is that nodes favor the longer of the two chains. It has more proof of more work. It's a little counter-intuitive so it bears mentioning that there is no loss of transaction data when a block doesn't make it into the consensus chain.