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No it is not normal for anyone to talk about suicide even once. Suicide amongst teens is high. Teens are very impulsive and it is possible to suicide even when one is only wanting to make a gesture of suicide.
Some red flags to look for would be history of depression, giving away possessions, describing a specific way/plan to suicide, writing about suicide in a journal/on line, etc.
Talk to him about it about, educate him on the treatment options (counseling, online support groups, etc)