Memory stick creating extra files while transfering data from iMac to PC

Just got new iMac and while I move files to USB memory stick everything looks okay. When I remove the stick and insert it to PC it shows double files keeping the originals and adding ._ in the beginning of each file. For example if the original file name is "Cover.png", the double file is "._Cover.png". Never happened before. I tried multiple memory sticks. Always the same. This means if I transfer for example 100 files in a folder from iMac, the result is having 200 files in the same folder on the PC. Means I have to erase all the extra stuff manually. Any ideas how to fix this problem?