If you see the error message "This item is already being downloaded and added into Chrome" when trying to install AdBlock, this is a known issue with Chrome. It can affect any extension, not just AdBlock.
Here's how to work around it:
- Type chrome://flags in the address bar. Find the text Enable Enhanced Bookmarks and select Enabled in the list below.
- Click Relaunch Now.
- You should now be able to install AdBlock from the Chrome Web Store.
- After AdBlock is installed, put things back the way they were. In chrome://flags, select Default under Enable Enhanced Bookmarks and click Relaunch Now.
If you don't see that option in chrome://flags, please give these general Chrome troubleshooting steps a try.