Should I write my name with an already gained bachelor title in another bachelor thesis?
The good news is that it doesn't matter. Very few people will read the thesis. If it is normative in your country to add B.Sc after a name, then you have the right to add it wherever you choose. This is not normative in the US or UK, where I have worked. However, there's no requirement that all titles be used, even in formal contexts. In sum: you can choose.