Plot multiple overwrites labels with evidences

When you call

result.plot_multiple(..., labels=labels, evidences=True,...)

the elements of labels are overwritten.

This means that if you're passing the same list to multiple calls to plot_multiple you append the evidence every time and it gets a little silly looking.

Can we avoid overwriting the list in this case? Maybe we can make a copy of the list at/near https://git.ligo.org/Monash/tupak/blob/master/tupak/core/result.py#L646?