Given two arrays like the following:

a = array([1,2,4,5,6,8,9])

b = array([3,4,7,10])

I would like the output to be:

c = array([1,2,3,4,5,6,7,8,9,10])

or:

c = array([1,2,3,4,4,5,6,7,8,9,10])

I'm aware that I can do the following:

c = sort(unique(concatenate((a,b)))

I'm just wondering if there is a faster way to do it as the arrays I'm dealing with have millions of elements.

Any idea is welcomed. Thanks