Overall, cognacy seems to be an advantage rather than a disadvantage. Thus learners of an L2 cognate to their L1 outperform those who learn a lexically alien L2 (Ringbom, 1987, in Granger, 1993):
— ❐ Going it alone﹕ self-instruction in adult foreign-language learning
- this would then be an example of "positive interference" if you so want...