I am following Ben Eaters 6502 project and build the clock and connected an w65c02 and the resistors on the databus for the NOP command (without eeprom or ram I'm still waiting for those components).
When monitoring the "program" running with an Arduino i noticed that the cpu executes each instruction twice.
Even if stepping manual through the program I must push the cock button twice before the cpu advances one instruction.
To be sure I connected leds to the adress lines d0..d3 so i could see the adress bus advancing and i have to push the clock button twice.
I checked all the connection multiple times and far as I can see everything is connected as it should.
I've tried shorter and longer clock pulses whitout succes, the pulses themself are clean, no rebounce whatsoever (impossible they come from 555 timer).
Is there something else i can try to solve this riddle?

It's not executing twice, the NOP instruction just takes two clock cycles to execute. That's just the way Ben's monitor works. He mentions this at 25:03 in his first 6502 video.

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x86?  We ain't got no x86.  We don't NEED no stinking x86!

Welcome, djenn! As noted, you'd be well-advised to look at a table of opcodes - how many bytes, how many cycles.

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http://WilsonMinesCo.com/ lots of 6502 resources
The "second front page" is http://wilsonminesco.com/links.html .
What's an additional VIA among friends, anyhow?

Thank you all for helping me out!
It's clear to me now, whitout looking at the instruction set I assumed that a nop instruction was a one cycle instruction.
And while I'm waiting for the memory chip i cannot run a real program to see the difference.
The 55 is slow indeed, I'll add an hc414 for to be sure.

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x86?  We ain't got no x86.  We don't NEED no stinking x86!

I meant to write that, i guess I was not fully awake.

It looks like someone modified the Arduino sketch (program) to monitor a new connection from the SYNC pin on the 65C02 so that the Arduino Mega will display the opcode mnemonic during the SYNC (opcode fetch) cycle.

link: https://github.com/dpm-343/6502-monitor

viewtopic.php?f=1&t=7117

These quick reference charts show you the clock cycles.

Note the caveat about this being NOT the 65c02.

But close enough for the purposes of understanding this concept.

The "C" column shows clock cycles for each instruction.