Different tax treatment makes up 1.5x by itself, and then you have all the overhead (accounting, insurance, legal - if you're too stingy to pay a proper lawyer and accountant in the beginning, that will cost you much more later) and "benefits" (e.g. vacation and sick days make a ~10% surcharge), so 2x hourly rate means that as a contractor your annual actual cash-in-hand for the same amount of work is roughly the same
if you have a pre-arranged contract where everything works perfectly, which it does not - you need to account for customer nonpayment, you need to account for hours worked on sales proposals that don't go anywhere, you need to account for time&effort in getting this customer, negotiating scope and terms, so if you're treating contracting/consulting as a proper business then an appropriate "customer-billable-hour" rate is 2.5-3x to "FTE-contract-rate" for the same person doing the same work.
If you get a good long-term customer that gets you lots of sustainable work on a retainer - reducing (but not eliminating) the overheads on sales,negotiation,downtime and legal/nonpayment risk - then you can go 2x; but 1.5x would be losing you money even in that case, you'd be financially be much better off taking the 1x full time job then.