Depending on the taxes, people certainly are taxed twice in certain situations - the most obvious being VAT, which is certainly an example of double taxation when considered with income tax.

There should be limited taxation on useful investment and massive taxation on hoarded resources (including land, gold, artworks, antiques, etc) and casino speculation.

Enhanced taxation on luxury spending wouldn't be a bad thing either.

But while we're in fantasy land, it would also be a good idea to disbar anyone with a net worth of more than £x million from political office, and to make it an offence for any corporation with a net worth of more than £y billion to spend money on lobbying.

These things will not happen, but most of our current problems are caused by corrupt money swilling around politics, and they'll only be solved once money and politics become separated.