Can I have a variable with repearted observations for each cross-sectional unit?

[John Andrews]

wrong question. How do I delete it

[Maarten Buis]

It is very common for panel models to control for the wave/period, i.e. to have indicator/dummy variables for wave 2 and 3. If you are doing that to, then you cannot add your variable x because that would be perfectly collinear with the wave variable.