Duplicate measurement keys don't raise exception

[viathor]

Found by @Strilanc in review of #2114 .

To reproduce:

In [1]: import cirq

In [2]: q = cirq.NamedQubit('q')

In [3]: c = cirq.Circuit.from_ops(cirq.measure(q, key='a'), cirq.measure(q, key='a'))

In [4]: c
Out[4]: q: ───M('a')───M('a')───

In [5]: sim = cirq.Simulator()

In [6]: sim.run(c, repetitions=5)
Out[6]: a=0000000000

[vtomole]

This needs to get in #1862.