The opening supplication - دعاء الاستفتاح - concludes with the words ولا إله غيرك , commonly translated as, ‘and there is no god [worthy of worship] other than You.’
The phrase لا إله though, is a much stronger than what this translation conveys. The type of لا at play here is a لا of absolute negation - it is the strongest denial of absolutely any object that falls within that category of word. And then we state an exception after the negation, which limits the meaning to only that thing that has been mentioned as an exception, and nothing else.
So when we conclude this supplication with the words ولا إله غيرك we are essentially saying, “O Allah, I affirm that there is absolutely, categorically, not a single deity, person or thing that is worthy of worship, other than You.”