Duplicate preset brushes in Lightroom Classic

Brush presets are stored as .LRTEMPLATE files in a Windows folder like-

C:\Users\{Your User Name}\AppData\Roaming\Adobe\Lightroom\Local Adjustment Presets

Exit Lightroom-Classic, open the folder in Windows File Explorer, and manually delete files you don't want.

The more 'painful' way of deleting brushes is in the Lr-Classic masking interface -

Create a Brush or Gradient mask, select a 'Brush' preset you want deleted, re-select the same Brush Preset (it will appear with a check mark), and at the very bottom of the brush list choose- [Delete Preset " selected brush"... ]