Double taxation between Germany and US for dual citizen

In general, when 2 countries have good diplomatic relations, it is very rare for there to purposesful cases of 'double-taxation', where you pay the same tax in each country as you normally would. Typically, you have the ability to claim 'foreign tax credits' in one or both jurisdictions, that reduce your other tax liability down somewhat. Mathematically, this is typically intended to result in you paying a combined tax equal to the higher of the two rates, with some going to each country.

For example, a US citizen living in Canada with a regular job would likely pay all their tax to Canada, which has the first right to tax employment income earned on its soil, and since Canada typically has higher taxes on employment income than the US. In a case where the total Canadian tax paid would be less than your US tax otherwise owing, you would pay what basically amounts to a 'top-up' payment of tax to the IRS, and when you add your Canada + US taxes paid, it would equal to the same as the amount of tax a US-located person would pay on the same income.

Note that the US is one of only 2 countries that taxes you as a citizen effectively the same as if you lived there [the other country is Eritrea]. For other citizenships, this situation only arises if you do something like live in one place half the year, or work offshore, or other specific circumstances.

In reality, to know the exact nature of taxes owing in your situation would depend on arrival / departure dates in Germany, type of income, and many other factors. Either commit to doing 10-20 hours of research yourself to get comfortable with how to do it yourself [start by researching 'tax treaties' between the US and Germany], or expect to pay probably 1-5k to a tax professional to sort it out for you. If you go the professional route, make sure in advance they commonly work with US immigrants living in Germany.

As @Grade 'Eh' Bacon wrote, an exact answer would take a lot of research.
However, as a rough overview: as a US citizen, your global income is taxable in the US, and you need to file your taxes to uncle Sam (one of the reasons many US citizen living permanently abroad abandon their US citizenship). Fortunately, any taxes already paid in Germany reduce your taxes in the US, and US taxes tend to be lower, so you'll end up with zero in most cases - just a lot of extra work.