KM is only doing what you tell it to do - it's not intentionally losing duplicate cells. So double-check to make sure the clipboard contains what you think it does.
What's happening is that Keyboard Maestro's clipboard history “helpfully” removes duplicate entries.
<soapbox>
Personally I think this is a terrible feature in that the user has no control over it – nor do I think it's documented in the manual.
The idea that a user would never need duplicate entries in the clipboard history is shortsighted.
My opinion is that the user should be able to either toggle this feature on/off – or that duplicates should always be accepted, but the user could at their discretion remove duplicates from the history.
Of course having both options would also be acceptable.
LaunchBar's clipboard history and Paste.app both play the same game, so this heavy-handedness is not limited to Keyboard Maestro.
</soapbox>
So, what's the solution?
I do NOT recommend dictionaries – they are too complicated to mess with for this task.
It depends upon how bad you want to use the feature – I'd just roll my own paste-pop with a Keyboard Maestro variable instead of the KM Clipboard History.
You don't show your entire macro, but here is my macro that works fine with Excel.
I did not test with Google Sheets.
The key is in copying the Excel data to a KM Variable, which is plain text, and then setting the Clipboard to that Variable, and then Paste.
It does NOT depend on the behavior of the KM Clipboard History.
Example of Macro Running
Below is just an example written in response to your request. You will need to use as an example and/or change to meet your workflow automation needs.
I have to copy a test answer key into an online testing platform that doesn't have an import function. Answers for each question have to be submitted one at a time. My answers currently reside in a spreadsheet, and I don't want to spend a day going back and forth between spreadsheet and test creation UI.
Here are the two macros my simple mind ended up using. Haven't been able to break it yet.
Macro 1 - Copy to File:
Macro 2 - Paste and Remove Line from File
I'm sure there is a better shell script solution using the sed command that doesn't require the creation of a temp.txt, I'm just not familiar enough with it.
What's nice about this system is that if I get pulled into another task, I can use my system clipboard and it's associated paste and pop functionality without losing my place in the other ordered clipboard.
There's nothing wrong with using the shell and files, but it's not necessary.
I'm using Keyboard Maestro variables and native actions for these.
If this task was mine I'd want to know if I could use the Set Front Browser Field to Text action to fill the form instead of using the clipboard for pasting.
As it's written, if an answer is more than one letter, this macro doesn't paste each cell as a whole. That is, the answer "ADE" gets pasted as "A", then "D", then "E".
I deleted some of the question marks from each regular expression and it works perfectly.