Forbid coercing multivariate polynomials with repeated variable name in some cases

[williamstein]

Magma and Sage let us do this:

> R<x,x> := PolynomialRing(RationalField(),2);
> R.1*R.2;
x*x

sage: R.<x,x> = QQ[]

sage: f = R.0*R.1
sage: f
x*x
sage: magma(f)
x*x
sage: gp(f)
x^2
sage: pari(f)
x^2
sage: singular(f)
x^2

Component: commutative algebra

Stopgaps: todo

Reviewer: Lorenz Panny, Samuel Lelièvre

Issue created by migration from https://trac.sagemath.org/ticket/10575

[williamstein]

comment:1

I think that gp(f) should raise an error, since gp can't represent a poly in two variables with the same name. Magma can fine.

[sagetrac-jakobkroeker]

Stopgaps: todo