This warning gets logged when you have multiple functions with the same name being exported to Excel. Because you can't have multiple functions with the same name in Excel, only the last one will be used.
For example, consider two Python modules:
# module1.py
@xl_func
def func():
....
# module2.py
@xl_func
def func():
...
module1.func and module2.func in Python are two different functions, but both can't be exported to Excel as the worksheet functions with the same name "func". The warning in the log file is to let you know this is happening.
To get around this, you can either renamed your duplicate functions, or use the "name" argument to @xl_func or @xl_macro. For example:
# module1.py
@xl_func(name="func1")
def func():
....
# module2.py
@xl_func(name="func2")
def func():
...
PyXLL will use the name argument instead of the function name, and so register the two Python functions as two distinct Excel worksheet functions.