XPath to find an element with a similar sibling

Suppose I have this XML:

<x>
    <e a='1' b='A'/>
    <e a='1' b='B'/>
    <e a='1' b='A'/>
</x>

I'd like to write an xpath to find any elements e which:

1. Have attribute @b = 'A'
2. Have the same value for attribute @a

The xpath can't reference the literal value of attribute @a, however. It can reference t开发者_运维百科he literal value of attribute @b.

Or more generally, I want to find if there are any instances where there are two or more elements e[@b=A'] with the same value for attribute @a.

Is this possible?

I solved my problem with the following xpath:

//e[b='A' and @a=following-sibling::e[b='A']/@a]