[Python] Find common characters between two strings

I am trying to print the common letters from two different user inputs. For example, if the inputs are "one" and "toe", the expected result is "eo".

I need to do it using a for loop.

I am running into two problems:

1. 
   My statement if char not in output... is not pulling unique values.
   
   
2. 
   The output is giving me a list of individual letters rather than a single string. I tried to split the output but split ran into a type error.
   

wrd = 'one'
sec_wrd = 'toe'

def unique_letters(x):
output =[]
for char in x:
if char not in output and char != " ":
output.append(char)
return output

final_output = (unique_letters(wrd) + unique_letters(sec_wrd))

print(sorted(final_output))


How can I do it correctly?

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