Solo Queue vs Duo

My cousin is shoving how he is on a win streak to gold 5 with his duo. In my hand, I am solo queuing doing the best I can to get to gold by myself. What I am saying is that if you solo and you are duo a lot, will you just lose more games because you duo? I am very confuse in the solo vs duo thing. Tyler1 said that if you duo, you are hardstuck and I don't if that's true.