input: Write a function to remove all duplicates from an array of strings and return the resulting array. The resulting array should have unique elements in the order of their first appearance.
  
  def removeDuplicates(arr):
   result = [] 
    
   # traverse for all elements 
   for i in arr: 
    
    # check if exists in result 
    if i not in result: 
     result.append(i) 
    
   # return result 
   return result
  
  assert removeDuplicates(['a', 'b', 'a', 'c', 'c', 'd']) == ['a', 'b', 'c', 'd']
  assert removeDuplicates([]) == []
  assert removeDuplicates(['h', 'e', 'l', 'l', 'o']) == ['h', 'e', 'l', 'o']
  assert removeDuplicates(['x', 'y', 'y', 'z', 'x', 'z']) == ['x', 'y', 'z']
  assert removeDuplicates(['a', 'a', 'a', 'a']) == ['a']
output: def removeDuplicates(arr):
   result = [] 
    
   # traverse for all elements 
   for i in arr: 
    
    # check if exists in result 
    if i not in result: 
     result.append(i) 
    
   # return result 
   return result