Should math.evaluate("f(x,x)=x") be an error?

[ChristopherChudzicki]

Describe the bug
f(x,x) = x parses and evaluates to a function with somewhat unexpected behavior.

To Reproduce
Run:

const math = require('mathjs')
const f = math.evaluate("f(x, x) = x")
f(1, 2)

I came across this while working on (the rewrite of) my app, math3d.org. See https://www.math3d.org/S9Kp3MDh5 for how this came up. I want user input of f(x, x) to be an error. It seems like something unintended/confusing by users. Certainly I can handle this as a consumer of the mathjs library.

But... how should MathJs handle this? Options:

1. The current behavior is correct: allow parsing and evluating f(x, x) = ..., with later parameter values taking precedence.
2. Throw an error during parsing
3. Throw an error during evaluation: can't determine which value of x is correct.

(2) Seems too restrictive. Parsing the expression can be useful for getting structural information. (1) and (3) both seem somewhat reasonable... Curious if this has come up before / is intended behavior.

[josdejong]

Thanks for bringing this up. It never occurred to me that you could enter the same variable multiple times, that should indeed throw an error. I've opened an issue to address this bug: #2818