IMCE stops uploading image in the middle of uploading

when my editors try to upload a new image, it stops uploading in the middle of the process and gives message: "A file with the specified name already exists.".
now when i check on the server, i found that it has uploaded part of the image only (image cropped). the editors are using google chrome and Firefox/windows operating system.
for me I am using MAC OS. and when i upload a new image, everything works fine with both FF and google chrome.
any idea is highly appreciated