Cvetinov, read the first page, my post is just widening of the first post in which you can find basic informations. However, I would say that double-cross is one of pre-Christian symbols just like simple cross.

A theory (by slovakian historians) says that the double cross arrived to the part of the Hungarian Kingdom (current-day Slovakia) …

Nobody says that, since no Hungary existed at that time (the 9th century).