Duplicate Emails

Dear Sir
I have an email account info@aboumerhilines.com , it was configured into 2 computers using OS XP SP2 and 1 laptop OS Vista Home premium . each Outlook in each computer was configured to recieve emails from info@aboumerhilines.com and i tick on Leave a copy on a server , to allow 3 users to recieve the same recieving email . they are working well but suddenly they recieved the same email several times each time they click on send/recieve bottom , i checked with my Email Server Support and there is no problem with him cause the other accounts are recieving well but only info they are recieving the same email several times , i change the reconfigure the Outlook Dll files in each PC , I repair the outlook , i change the profile , and I do everything , but i still recieve the same email on each computer several times .