Marketing to the same house question

Hi. I have a dilemna but maybe it isn’t a dilemna after all. Tell me what you think.

I have been finding ugly vacant houses by driving the neighborhoods and am currently marketing to some ugly vacant houses in a particular neighborhood that is really hot in my area. I have noticed a lot of investors in this area. I am on an investor email list and a few times I have seen their emails come out with houses that they have under contract which are the same houses I have spotted and am doing marketing and phone calls to. I don’t want to step on another investors toes but at the same time I have no clue that I am chasing down a lead on a property that another investor has under contract. How can I avoid this problem in the future if I see a potential wholesale in this area and how would I know if the house is under contract with another investor (a lot of investors don’t file the memorandum of agreement). Should I even worry about this?