How to prevent a module from being imported twice?

Python modules aren't imported multiple times. Just running import two times will not reload the module. If you want it to be reloaded, you have to use the reload statement. Here's a demo

foo.py is a module with the single line

print("I am being imported")

And here is a screen transcript of multiple import attempts.

   >>> import foo
   Hello, I am being imported
   >>> import foo # Will not print the statement
   >>> reload(foo) # Will print it again
   Hello, I am being imported

Imports are cached, and only run once. Additional imports only cost the lookup time in sys.modules.