What kind of normalization rule does this violate?

Question

Suppose I have two tables on a database, T₁₀ and T₁₁, having 10 and 11 columns, respectively, where 10 of the columns are exactly the same on both.

What (if any) normalization rule am I violating?

Answer 1

Edit: I have been informed that no Normal Forms are violated here in theory. Since this was the accepted answer, I'm leaving it here for reference, and because thinking about 3NF may in practice help avoid situations like that in the question.

You are violating the Third Normal Form (3NF), because if mostly the same data is held in both tables, then every attribute of each table is not directly dependent on the key of its respective table.