I have come across two opinions on this:
This was his actual name
His actual name was Suyodhana and he was later on known as Duryodhana.
What was Duryodhana's actual name?
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Both are names for the same person and often used interchangeably. Suyodhan means one who is good with arms (in warfare). Duryodhan means unconquerable warrior. Naturally there can be many who are good in battle but there can be only ONE who is unconquerable, by definition. Duryodhan was more unique and it stuck.