This issue is caused by various improvements we have made to the Blocs Extension Manager that require older Brics to be recompiled in a newer version of Blocs.
1. Navigate to the following directory on your Mac:
~/Library/Application Support/Blocs 3/brics/
2. Within the above location you will find a list of directories, one for each of the custom Brics that you currently have installed. The custom Bric that is shown multiple times in the Bric Bar should be obvious, as there will be multiple directories with the same name but with a numerical suffix appended to the name e.g. picture-bric, picture-bric-2, picture-bric-3.
Proceed to delete all of the directories for the problematic custom Bric, taking care not to delete any directories for other Brics that are correctly installed. At this stage, if you are unsure, we recommend that you make a quick backup copy of the entire brics directory.
3. Now you have removed the directories relating to the problematic custom Bric, try to reinstall the Bric from fresh, using the Extension Manager. If the Bric fails to show in the Extension Manager then there is a problem and you should repeat the above steps and contact the developer of the custom Bric.