I am currently dating withthis chick who I am sure is leading 2 relationships at the same time in different places. When we started dating she said of being single butnow I have the security she is back in her last relationship as well. I have tried approaching the subject subtly with her but there has been no way of her admitting anything.
Does anyone has any suggestion on how to lead this?

Well, as I see it, you've got 2 options. Either you accept that she wants to have multiple relationships and be cool with the relationship that you DO have with her. I usually suggest that sort of thing for guys who themselves desire to, or currently do have multiple girlfriends of their own. Otherwise...

If you want an exclusive relationship you end up either having to get her to choose you over him, or ditch her. In order to get her to ditch him you need to make her realise you're the better choice and that YOU won't date her if she isn't gonna be exclusive because you're not into that sort of thing; when you're actually dating someone you expect the relationship to be exclusive. If that isn't enough to go on then let me know.