Python - Replace duplicate Occurrence in String

Last Updated : 07 Nov, 2025

In this article, we will discuss how to replace only the duplicate occurrences of certain words in a string that is, replace a word from its second occurrence onwards, while keeping its first occurrence unchanged.

Example:

Input: Gfg is best. Gfg also has Classes now. Classes help understand better.
Output: Gfg is best. It also has Classes now. They help understand better.

Using list comprehension + set

This is the cleanest and most efficient approach. We use a set to keep track of seen words and a list comprehension to perform replacements in a single line.

Steps:

Split the string into individual words.
Iterate over each word.
If it exists in the replacement dictionary and hasn’t been seen yet, keep it as-is.
If it appears again, replace it with the mapped value.
Join the modified words back into a single string.

Python

s1 = 'Gfg is best . Gfg also has Classes now. Classes help understand better .'
rep = {'Gfg': 'It', 'Classes': 'They'}
seen = set()
res = [
    rep[word] if word in rep and word in seen
    else (seen.add(word) or word)
    for word in t1.split()
]
s2 = ' '.join(res)
print( s2)

Output

Gfg is best . It also has Classes now. They help understand better .

Explanation:

split(): breaks the string into words.
set(): tracks words that appeared once.
(seen.add(word) or word): ensures first appearance is stored but not replaced
' '.join(res): combines words back into a string.

Using Regular Expressions

This approach uses regex patterns to detect repeated occurrences of target words and replaces them using re.sub().

Steps:

Import the re module.
Define a regex pattern to match the target words from the replacement dictionary.
Use a function in re.sub() to perform conditional replacements based on previous occurrences.

Python

import re

s1 = 'Gfg is best . Gfg also has Classes now. Classes help understand better .'
d = {'Gfg': 'It', 'Classes': 'They'}
pattern = r'\b(' + '|'.join(re.escape(k) for k in d.keys()) + r')\b'
seen = set()

def fun(m):
    word = m.group(1)
    if word in seen:
        return d[word]          
    seen.add(word)
    return word                  

res = re.sub(pattern, fun, s1)
print(res)

Output

Gfg is best . It also has Classes now. They help understand better .

Explanation:

\b(...)\b: matches complete words only.
re.escape(): safely handles special regex characters in keys.
re.sub(pattern, fun, s1): calls fun() for each match and replaces only later duplicates using a seen set.

Using split() + enumerate() + Loop

This is a more explicit and beginner-friendly approach using loops and sets. It manually iterates through the words, tracks first occurrences, and replaces duplicates.

Python

s1 = 'Gfg is best . Gfg also has Classes now. Classes help understand better .'
rep = {'Gfg': 'It', 'Classes': 'They'}
words = s1.split()
seen = set()
for i, word in enumerate(words):
    if word in rep:
        if word in seen:
            words[i] = rep[word]
        else:
            seen.add(word)
res = ' '.join(words)
print(res)

Output

Gfg is best . It also has Classes now. They help understand better .

Explanation:

s1.split(): splits text into tokens.
if word in rep and word in seen: replaces repeat occurrences using mapping.
seen.add(word): records first encounter.

Using keys() + index() + List Comprehension

This method uses the list.index() function inside a list comprehension to find if a word has appeared before. It replaces only the repeated words and keeps the first one as it is.

Python

s1 = 'Gfg is best . Gfg also has Classes now. Classes help understand better .'
d = {'Gfg': 'It', 'Classes': 'They'}
words = s1.split()
res = ' '.join([
    d.get(word) if word in d and words.index(word) != i else word
    for i, word in enumerate(words)
])
print(res)

Output

Gfg is best . It also has Classes now. They help understand better .

Explanation:

rep.get(word): retrieves replacement from dictionary.
words.index(word) != i: ensures only later occurrences are replaced.

Python | Replace multiple occurrence of character by single
Remove all consecutive duplicates from a string

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Article Tags :

Python - Replace duplicate Occurrence in String

Using list comprehension + set

Using Regular Expressions

Using split() + enumerate() + Loop

Using keys() + index() + List Comprehension

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