How to find List has duplicate values in List [duplicate]

Try to use GroupBy and Any like;

lstNames.GroupBy(n => n).Any(c => c.Count() > 1);

GroupBy method;

Groups the elements of a sequence according to a specified key
selector function and projects the elements for each group by using a
specified function.

Any method, it returns boolean;

Determines whether any element of a sequence exists or satisfies a
condition.