Models duplicate in customized Themes

Hi,

I created a customized theme.

From a Blank theme, I have deleted all the existings slides(models) from the theme.

Keep Title 1 and duplicate from it for my new models.

In the presentation, when I copy or duplicate a slide,

it duplicates the model as well. I have 2 models Title 1...

Does anyone could tell me if I did something wrong

or it's a bug...

Thank you for helping

Gregoire