You need to use parentheses around the xpath expression and from there use the index.

expr = "(//button[@name='action_rfq_send'])[1]"

expr = "(//button[@name='action_rfq_send'])[2]"

expr = "(//button[@name='action_rfq_send'])[3]"

I don't know how to do it with one expression, but you can do this:

<xpath expr="//form/group[1]" ...
<xpath expr="//form/group[2]" ...
<xpath expr="//form/group[3]" ...

In my case using numbers fails.

Just repeat the same expression as many times as you need:
<xpath expr="//button[@name='action_rfq_send']" position="after"></xpath>
<xpath expr="//button[@name='action_rfq_send']" position="after"></xpath>

The Xpath code will be such as:

<xpath expr="//button[@name='action_rfq_send'][1]" ...............>

<xpath expr="//button[@name='action_rfq_send'][2]" ...............>

<xpath expr="//button[@name='action_rfq_send'][3]" ...............>