How do you pronounce the letter "W" ???

[Deuem]

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[like2learn]

Here Ray Ashley pronounce it like double you: RapidSteps

[Raikun]

It is "double-U"
But your friend is correct, when speaking quickly we do slur our words. However, it's not Doub-Lu.

It is acceptable when speaking fast to say "Dubba U" or "Doub U"

Like when you read out an address, you would say "Dubba U Dubba U Dubba U DOT using english DOT com" for UsingEnglish.com - Learning English (ESL) Online

Hope I've helped!