Since Appian is case sensitive.
So my requirement is if I select a user "Radha" then it gets created but again if someone tries to create a username in lowercase"radha" then it should not get created.
Do suggest how to check it.
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You'd probably be interested in the function isUsernameAvailableForNewAccount() found in the Personalization Utilities plug-in. Beyond that, if some usernames in your system will be hand-entered by other users, I strongly suggest forcing them to either all lowercase or all uppercase, to remove some of the guesswork.
Thanks..