Why doesn't the copy function ever know that 2 folders are identical?

I'm copying folder A on external drive E to folder B on external drive K. Folder B is an older copy of folder A which I now want to update so that I have an identical copy of folder A on drive K. I'm calling the copy of folder A on drive K "folder B" just for simplicity. Folder A is 27 gigabytes, 121 folders, 4,000 files.

When I copy A to B the system copies just the new files and after awhile tells me that there are copies of the remaining files already in folder B so I click on cancel or skip remaining files. Then just to be sure I copy folder B back to folder A and it finds a couple hundred files that it says are not in folder A so it adds them then tells me the rest are already in A. So next I just want to be really sure they are both the same so copy folder A back to folder B again, but it starts copying another hundred files from A to B.

I have done this back and forth thing over 20 times till I got completely bored doing it and every single time it copys more files from B to A, then more files from A to B, then more files from B to A, then more files from A to B and on and on. If I look at the properties of both A and B they are identical, same number of bytes, files and folders. But the copy process never sees that they are identical. It will forever think there are some files in A that are not in B and that there are some files in B that are not in A no matter how many times I do this cross copying.

I realize I can just check properties earlier and let it go at that when the properties are identical, but since I keep multiple backups of important files and I'm not always sure when they were last updated I like to do this copying in both directions. But now I find that I can't rely on Windows 7 to really know that some files already exist and it doesn't need to keep copying them over and over again. Is this some kind of bug in Windows 7?

Thanks, Mitchell. I'll give it a try.

While I think the SyncToy is a good tool, I didn't find that it helped me because it can only deal with one folder at a time. By highlighting and dragging I can copy dozens of folders at a time. It would take many hours to do with the SyncToy what I can do by dragging and dropping in a matter of minutes. I guess I will just remain mystified as to why the copy function done this way does not recognize that it has already copied some files. I may try clicking on refresh to both open windows after the first copy and see if that makes a difference, or even closing and reopening the windows. I have found many times in the past when that has solved some problems related to Windows not recognizing all the files that are supposed to be in a folder.

If you attempt to copy a file to an identical copy of that file, it will tell you that it already exists and only offer to let you replace it.

If you attempt to copy a file to a similar but slightly different copy of that file, it should offer to Merge it which will then sort out the file differences letting you choose based on the size and age differences.

So it's OK to accept Merge if offered because you still get control over what ends up being in the copied location as you can choose between the differences.

If this is not what you need exactly then let me know.