# Taylor expansion twice for a general function cause problem?

Hi,

I met a problem when expanding a general function twice:

```
y = var('y')
f = function('f',var('x'))
g = f(x=var('eps')*y)
h = taylor(g, eps, 0, 1)
taylor(h, eps, 0, 1)
```

The last expression ends up with an error: NotImplementedError: arguments must be distinct variables

Is it a bug or I got anything wrong? Thanks!