get_template_part() calls locate_template(), and both functions are running file_exists() checks for the same file over and over. There is no caching. Meh.
But … PHP has an internal cache for file look-ups, so a direct file access will not happen on every call.
There is an edge case, most developers aren’t aware of: A file can be deleted after the first access, and file_exists() will still return true if PHP’s cache hasn’t updated.