Using Zen Cart Version 1.3.8a

I tried to install Dual Pricing Version 1.7 and received error:
"Cannot redeclare zen_update_products_price_sorter..."

What does this relate to?

Am I using the correct version of Dual Pricing?

I also tried uploading the previous version 1.6 and got the same error.

I have done a complete restore of the site and database to get me back to start point.

All OK... but I would like to use this module.

Any help appreciated.