Problems with boolean operation

Hi, I have some problems with union boolean operation:
when i try to make the union of two or more selected objects (‘w’ key + Union option)I get odd results, such as the duplication of one of them or the duplication of random combinations (first with second, second whit third…).
Am I the only one or does somone else have the same problem?

P.S. Blender version: 2.41 (the last one…I suppose)

…I found my answer looking at similar posts, I’m sorry, I’ll search before posting…