This seems like a problem you could answer rather easily if you wanted to think a little bit....
In case you aren't familiar with it, the formula for the number of diagonals in a polygon with n sides is (n(n-3))/2. (n-3 diagonals from each of the n vertices; each one gets counted twice, once from each end.)
Given that formula, you could simply try different small values of n and see if you can find one that satisfies the condition.
Or assuming you know a little algebra, you can solve the equation that says the number of diagonals is twice the number of sides: