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.
First try restarting Chrome. If that doesn't help, restart your computer. And if that doesn't help, try this workaround:
- 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.