Reapplying to jobs

Is there an appropriate length of time to wait before sending one's resume to a company that one has already sent one's resume. In my case, I saw ads for positions for this company a couple months ago and sent my resume through the website. Aside from acknowledgement that they received my resume, I have not been contacted by these companies. These companies are positiong ads for positions again. Should I send my resume again? I don't know if 1) These jobs were real positions 2) They are planning an expansion or mass retirement in the near future and are still recruiting for the same positions in the original ad 3) They have turn over and hired some people from the first ad but still need more people 4) I will never be good enough for them to even do a phone interview with.
I am thinking that I am probably not what they want. If I do have a chance at getting an interview though, I don't want to screw it up by looking dumb by applying to the same job twice. On the otherhand, I don't want to lose my chance because they discarded all the resumes received from the earlier ads.

Maybe the same position in multiple sites or multiple positions that have the same requirements.

Change your resume several times. Sometimes they just search for the maximum repitition of key words revolved around the position to catch their eyes in the search engine. They probably don't look at many of the resumes save their the one their filter lets through.

They are also well known companies and have posted on a couple of boards for these positions. I suppose it isn't like they aren't only getting 20 resumes and would automatically discard the 2 that are from the same person.
On a related note, if the company is posting a few jobs that I might be qualified for and interested in, should I apply separately to each one or should I note in the cover letter that I would be interested in the other positions as well.