Stop matplotlib repeating labels in legend

Here is a very simplified example:

xvalues = [2,3,4,6]

for x in xvalues:
    plt.axvline(x,color='b',label='xvalues')

plt.legend()

The legend will now show 'xvalues' as a blue line 4 times in the legend. Is there a more elegant way of fixing this than the following?

for i,x in enumerate(xvalues):
    if not i:
        plt.axvline(x,color='b',label='xvalues')
    else:
        plt.axvline(x,color='b')