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Overview: This article explains how to troubleshoot a common custom function error in Excel that may occur when loading the Active Workpapers add-in.

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Users may occasionally see the following error at the bottom of Excel during startup of the Active Workpapers add-in: Error installing functions. This add-in wasn't installed because a custom function with the same name already exists.

Excel requires all custom function names to be unique across all installed add-ins. This error can be triggered by:

Another installed add-in containing a function with the same name.

An old version of the add-in cached by Excel.

Hidden or inactive add-ins still registering functions.

Excel session not fully restarted after removing or updating an add-in.

- Another installed add-in containing a function with the same name.
- An old version of the add-in cached by Excel.
- Hidden or inactive add-ins still registering functions.
- Excel session not fully restarted after removing or updating an add-in.

IMPORTANT: If your Active Workpapers taskpane has successfully loaded, this error can be safely ignored. It does not impact the usability of the add-in.

If you wish to still resolve it, follow these steps to reset custom functions in Office 365 Excel:

Make sure all Excel windows are closed.

Check Task Manager (Windows) or Activity Monitor (Mac) to ensure no Excel processes are running.

- Make sure all Excel windows are closed.
- Check Task Manager (Windows) or Activity Monitor (Mac) to ensure no Excel processes are running.

Step 2: Clear the Custom Functions Cache

Office 365 caches custom functions, which can cause the conflict. Here is how to clear them:

Press <code>Win + R</code> to open the Run dialog.

%LOCALAPPDATA%\Microsoft\Office\16.0\Wef\

You’ll see some folders/files. Delete everything inside this folder (this clears cached add-ins and custom functions). See Microsoft’s guide for additional steps: <a href="https://learn.microsoft.com/en-us/office/dev/add-ins/testing/clear-cache#clear-the-office-cache-on-windows" rel="nofollow noopener noreferrer" target="_blank">Clear the Office cache on Windows</a>

1. Press <code>Win + R</code> to open the Run dialog.
2. Paste this path and hit Enter:
 %LOCALAPPDATA%\Microsoft\Office\16.0\Wef\
3. You’ll see some folders/files. Delete everything inside this folder (this clears cached add-ins and custom functions). See Microsoft’s guide for additional steps: <a href="https://learn.microsoft.com/en-us/office/dev/add-ins/testing/clear-cache#clear-the-office-cache-on-windows" rel="nofollow noopener noreferrer" target="_blank">Clear the Office cache on Windows</a>

~/Library/Containers/com.microsoft.Excel/Data/Library/Caches/

Delete any folders/files related to your add-in or <code>OfficeWebAddins</code> cache.

1. Open Finder.
2. Go to:
 ~/Library/Containers/com.microsoft.Excel/Data/Library/Caches/
3. Delete any folders/files related to your add-in or <code>OfficeWebAddins</code> cache.

Step 3: Remove Duplicate or Conflicting Add-ins

Open Excel → Home → Add-ins → More Add-ins.

Ensure the Active Workpapers add-in is not installed twice (e.g., admin-managed and individually acquired from store in my add-ins screen).

Remove or disable any other add-ins that may conflict.

In excel check File → Options → Add-ins for any other unused add-ins (including disabled or inactive add-ins) that may still register functions. Remove the addins that are not being used.

- Open Excel → Home → Add-ins → More Add-ins.
- Ensure the Active Workpapers add-in is not installed twice (e.g., admin-managed and individually acquired from store in my add-ins screen).
- Remove or disable any other add-ins that may conflict.
- In excel check File → Options → Add-ins for any other unused add-ins (including disabled or inactive add-ins) that may still register functions. Remove the addins that are not being used.

This ensures the cache is cleared and there are no conflicting functions.

- Open Excel again.
- This ensures the cache is cleared and there are no conflicting functions.

If the issue persists, uninstall and reinstall the Active Workpapers add-in: Home → Add-ins → More Add-ins.

- If the issue persists, uninstall and reinstall the Active Workpapers add-in: Home → Add-ins → More Add-ins.

How to troubleshoot error 'Error installing functions. This add-in wasn't installed because a custom function with the same name already exists.'

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How to troubleshoot error 'Error installing functions. This add-in wasn't installed because a custom function with the same name already exists.'

Issue

Cause

Resolution

Step 1: Close Excel Completely

Step 2: Clear the Custom Functions Cache

Windows

Mac

Step 3: Remove Duplicate or Conflicting Add-ins

Step 4: Restart Excel

Step 5: Reinstall Add-in (if needed)