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Your LimeSurvey version: 5.5.2+230109
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Survey theme/template: Fruity
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We plan to carry out repeated data collection on the same respondents, i.e. a panel survey. Specifically, we will interview the same respondents 4 times. I wanted to ask whether it is possible to have the same ID for respondents in all surveys, so that we can then simply merge the results of each survey?

A related question is whether it is possible to make any changes to the surveys (e.g. change questions, add some, etc.) or does the repeat survey have to be exactly the same?

We will have a lot of respondents (about 4000), so I would need everything to be as automated as possible so that no mistakes are made.

Thank you very much for your answers and any experience with this type of research in LS. 

Usually this id is provided by the panel provider..
So each person should have a unique id 

And you have to capture this id in your survey and send it back by the end links. So they are paid.

Thank you for your reply. However, there is no panel provider, respondents via emails will be entered into the system by us and have no remuneration for doing so. So when I enter them via email, I will assign them an ID which will remain for the duration of all the surveys?

If you have the email,
do it as a token based survey.
And read the manual about "survey iteration"