strange error: "system error: Item has already been added..."

Sorry, meant to include sys info: Windows 11 Pro, Mathcad Prime v11.01

You obviously had found a solution for your problem yourself.

For future questions keep in mind that its usually hard to impossible to debug just a picture. So whenever possible you should attach your worksheet or, if that's not possible, create a sheet with dummy data which shows the same problem you experienced. It's also advisable to say which version of Prime you are using, especially if its not the latest, current version.

In case of your screenshot the only thing I could notice is that the "ns" in the function argument is not labelled as "Unit" and I would have suggested to do so manually.

But the error message in your picture is quite strange and leave me with no clue what the cause could have been.

BTW, I noticed the gaps in your plot. So I suspect that Qrr is a function created by parametrising a solve block or by using the "root" function. Using a different guess value (or even a guess value dependent on the time argument) may help.

Did you keep the sheet which showed the error? If yes and if you are allowed to share it I would ask you to attach it here, would be interested in this error.