If you have a lead with the same contact as an existing contact it will completely delete the record upon conversion of that lead. One incident of mine I had 5 companies all of which had the same company and they were in the system as leads. I was able to convert 1 of them with no problem, I could go back and view the leads with all of the information but immediately after converting the lead all of the information disappeared and was unrecoverable for all of the remaining companies.
According to the support staff this is not a bug but rather a normal behavior in the CRM as to when you have 2 Leads with the same email address and information but have completely different companies. Once you convert the 1st Lead, it will create the People record and the Company record if it is not yet in the list of your Company records. However, when you convert your 2nd Lead with a different company, this Lead will just be merged to the People record(the lead that was converted first) and will disregard the Company record from the 2nd Lead.
I would like this to be fixed in some way. The best option would be to have it convert everything over but at the very least I would like it not to erase of the information we have been working on and have some way to convert it!!!