Double taxation & gross receipts tax

Why am I paying taxes twice on the same income?

You're not!

Income taxes and gross receipts taxes are different tax programs. Income tax is paid on the profit a business earns; gross receipts tax is paid on the sale amount.

Also read The taxes self-employed people pay.

I earn commissions on sales on which the company I represent pays GRT. Aren't taxes collected on them twice?

Commissions involve 2 separate transactions which may be subject to gross receipts tax. First is the sale of a product or service on which the seller must determine whether the sale is taxable. Second there is the commission paid to the sales representative for facilitating the sale. Note: If the sale is not taxable, then the commission is also not taxable.

Also read Non-taxable transaction certificates