Problem with calc with coupon

im using version 1.3.8a

This is what is happening:
Sub-Total: €289,00
Free shipping (Free Shipping Only): €0,00
TAX 19%: €46,14
coupon: -€50,00
Total: €431,86

When im not using the coupon its like this:
Sub-Total: €289,00
Free shipping (Free Shipping Only): €0,00
TAX 19%: €46,14
Total: €289,00

This is correct for me this output. The customer see the price of 289 euro en i diplat the tax of 19% that whats in it. So im showing the price from the beginning till the end what they have to pay.

Thanks in advance for the help and read
Hop someone can help me with this.

Greets
TALES

By looking little bit furder i think this is not the coupon file or mistake or bug. But its the ot_gv.php file that does this. but some how i can not find the problem what cost this.

To me whats happening is this the amount is 289 this is doubled = 578 - tax 46,14 - 100 = 431,86

the coupon was the amount of 100 and that person gives 50 to the other person but some how the 100 is back in the calculation.