I think when two languages are too similar to each other (but not the same), it can cause problems when learning. I tried learning Portuguese after learning Spanish in HS. Big mistake. The two languages are so similar that I end up mixing them up when writing.

There were also situations when I thought I knew how to say something in Portuguese (because that's how it's said in Spanish and Portuguese has similar words), but it turned out that that phrasing isn't used at all.