Trump has repeatedly raised doubt over whether he would accept defeat in November's election.
German Foreign Minister Heiko Maas said Sunday that an "unscrupulous" President Donald Trump was trying to sow doubt about the US presidential election by urging his supporters to vote twice, which is illegal.
"We owe an incredible number of things to the United States and the country remains one of our closest partners but... it is disturbing to see that an American president thinks he might need such" a move, Maas told Sunday's Bild, Germany's top selling daily.
"I have confidence that Americans' good sense will scupper this unscrupulous effort to sow doubt on the validity of the election with the later aim, probably, of not accepting defeat," he added.
Trump said earlier this week that voters could use a mail-in vote -- which he has slammed as a ruse to rig the election against him -- and then also cast a vote at a polling station as a guarantee, with officials left to decide which of the ballots to count.
The president has repeatedly raised doubt over whether he would accept defeat in November's election, charging that his Democrat Party opponents are doing all they can to fix the outcome.
Relations between Trump and the German government have become increasingly strained over a whole series of issues, especially defence spending, leaving the traditionally strong allies drifting further apart.
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