These figures have the same shape.
These figures have the same shapes.
These can both be taken as being correct. Both use the definite article the, which can be applied to both singular and plural forms, unlike the indefinite article an used below.
In the first, each figure has the same shape as all the other figures. Each figure in the set has the same shape. Thus these figures have the same shape.
In the second, when you look at the shape of each figure, and look at the collection of shapes, then you find that all the shapes are the same. The shapes of the figures are all the same. Thus these figures have the same shapes.
These figures have an identical shape.
These figures have identical shapes.
In this case, the difference is in the use of the indefinite article an in the first example.
The indefinite article an (or a) is used only before singular forms. So like in the first case above using the definite article the, the sentences can be rewritten in various forms to convey the same meaning, but when using the indefinite article a / an only the singular form is correct.