Cannot access the file because it is being used by another process

You don't offer any clues as to why there could be more than one process or user trying to simultaneously access a common file.  Regardless, once you figure out what is causing the simultaneous access, you are going to need to make use of concurrent locking or otherwise ensure no two tasks or users can access the same file.

A common example of how you can run into the problem you are seeing is if let's say you have a Custom Calculate Data Managment step that is supposed to write some data to a common [FilePath].  That will work perfectly if you only ever run the package for a single Data Unit at a time, or each DU writes to a unique file.  But if run for multiple DU's, you now have the risk of multiple parallel threads all trying to hit the common [FilePath] at the same time.  The solution in this instance is to make use of a OS Global lock when updating the common [FilePath], so that only one thread can update the file at a time.  OS Global locks are automatically dropped should a task fail, and remove any Global data when the overall controlling process completes or fails, making them pretty easy/low risk to work with.

If you have multiple different users or different processes that could be trying to hit the same common [FilePath], then OS Global locks will not work, as they only work for tasks sharing the same overall task.  At that point, I would suggest writing a "lock" row to a custom SQL table, where if the row exists, you are to treat it as locked.  This option gets trickier, as you need to consider what happens if a task fails or hangs before it releases the lock (deletes the row).