I am wondering if I can inject multiple OP_RETURN outputs in one transaction?
Can inject multiple OP_RETURN outputs in one transaction?
At the protocol level, absolutely nothing prevents you from doing this. However, such a transaction would not be considered a 'standard' transaction and would not be relayed (propagated) through the network.
Let's say, though, you were a miner yourself. Then you could mine a transaction with 100 OP_RETURN outputs in it, and no one would reject your block because of it.