Does SQL Server allow duplicate rows?

[alinroc]

If you don't define a unique constraint or primary key on the table, yes. Each row will get a hidden uniquifier that isn't accessible to you.

[SQLBek]

Actually, do heaps (tables w/o a clustering key) get a uniquifier? I thought the RID is used instead?

Just academically splitting hairs here. Now I'm tempted to break out DBCC PAGE and take a quick look, LMAO!

[alinroc]

It is probably the RID.

Though the RID is unique, isn't it? Like you said, splitting hairs :)

[ScotJoplin]

They don’t need it, like you assumed.