Hachik0

So, i've been pretty new to modelling. And i've found Sketchup the first way of making simple models and such. It is great so far, but i have an issue. When importing my model in to leadwerks, when i make a more complexed model, it causes to do this: And this is how it looks like when in leadwerks: You can clearly see lines overlapping. What is causing it? Did i do something wrong? Thanks!