Python - Replace duplicate Occurrence in String

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:

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

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:

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

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.

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.

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.

 Related Articles:

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