This is the first time I am visiting London
Or
This is the first time I have visited London.
I have seen both. I don't quite understand the difference.
They can be used interchangeably, and in casual speech the second verb is contracted: "I'm" and "I've". Both are formal-sounding by American standards. )
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They can be used interchangeably, and in casual speech the second verb is contracted: "I'm" and "I've". Both are formal-sounding by American standards. You'd typically hear instead something like: "This is the first time I've been in London." (The word "visiting" is 3 syllables, and it's rare to use a 3-syllable word in casual speech if a shorter one is available.)
Use the second one. The first one is not a native-sounding English sentence.