How can I display the price field twice?
You should use Commerce Extra Price Formatters module. 

Extra Price Formatters Implemented So Far


RRP / Your Price

Allows you to display the Retail Price and the User Price
for a product and optionally the amount the customer
will save.

Allows you to switch the display back to a single price if
there is no discount available on a particular product.
Formatted Price with Prefix and Suffix

Allows you to present a price in the style just £9.99 ex
VAT or only £9.99 for a limited time only.
Formatted Price with no decimal places

When you are selling items with whole unit prices, you
don't necessarily want to see £10.00. This formatter
lets you change the display to show £10. (use this with
care!)


There's a video demo here: http://www.youtube.com/watch?v=-WR3mKjou8M

Then you should add two price fields in your product content type. In your manage display tab you would be having a settings option to calculate your price excluding or including tax and other parameters and thus you can add two price fields.