Relabeling creates inconsistent BQMs

[JoelPasvolsky]

Description
Relabeling can create BQMs that are inconsistent with generated BQMs:

>>>  bqm = dimod.BinaryQuadraticModel({'a': 1, 'b': 2}, {('a', 'b'): 1.5}, 'BINARY')
>>> bqm.relabel_variables({'a': 'c', 'b': 'c'})
>>> print(bqm.linear, bqm.quadratic)
{'c': 2.0, 'c': 2.0} {('c', 'c'): 1.5}

In comparison:

>>> bqm = dimod.BinaryQuadraticModel({'c': 1, 'c': 2}, {('c', 'c'): 1.5}, 'BINARY')
>>> print(bqm.linear, bqm.quadratic)
{'c': 3.5} {}

Expected Behavior
Consistent treatment of a variable set in multiple places. For the given example, it's noticeable, but in relabeling large numbers of variables in a big BQM, this might not end well

Environment
dimod version here is 0.10.10

[arcondello]

Also {'c': 1, 'c': 2} in your second example is just {'c': 2}. You can get something similar to what you're looking for with bqm.contract_variables('a', 'b').