In my opinion, it is usually advisable to invest in tax deferred accounts before investing in a Roth, but I will put that issue aside and assume you are not asking about that.
Of course nobody knows whether Congress will make Roth IRA withdrawals taxable in the future, but I think there is a real possibility of that happening.
You asked how would they tax money twice. Are you kidding?
Where is it written that the government can only tax money once? Let me cite just a few examples.... you earn money and it is taxed (once), then you buy stuff, and the government charges you sales tax (twice), or, you pay your cell phone bill and you pay various FCC taxes (twice), or you buy property and they charge document stamps (twice). Also consider that when a corporation earns a profit, it pays taxes, and then, when the profit is distributed to you in the form of a dividend, you pay tax on the same profit...again. Also consider that you pay tax on earnings in the form of social security taxes, then, when you collect social security, you pay taxes on the social security benefits! I could go on, but you get the point, there is no such law anywhere that says the government can't tax you twice or three times on the same money.
P.S. Do you recall the main theme of President Bush's campaign in 1988? (Hint: it was "No new taxes!") Guess what he did after getting in office? He raised taxes. The point is... promises made can be promises broken.