How is a transaction not present in 2 or more solved blocks? If node A picked 10 unconfirmed transactions to solve in a block and node B picked the same 10 transaction in A plus another 10 transactions to solve. If node A finished solving the block informed all nodes and placed the block on the chain, what happens to the same transactions being mined by Node B?
When node B gets informed about block A being mined he would start working on a new block that does not conflict with the first one.

If block B happens to be mined around the same time as block A there would be a fork. All nodes in the network that hear about both blocks will keep both branches and will wait for the conflict to be resolved.

The miner that mines the next block will have to place his new block on top of one of the branches. When that happens and the new block gets broadcasted, all nodes will pick the branch with the most work and the other branch will become orphan.