I use Duck Duck Go as my primary search engine, but I often need to repeat a search on Google – Duck Duck Go is good, but nowhere near as good as Google for some searches, unfortunately.
I created this bookmarklet to prepend "!g" to the search and re-submit the search form. This will automatically perform the same search on Google.
If you use Safari and place it near the beginning of your bookmarks bar like I did, you can use ⌘3 (in my case; it's the third bookmark from the left) to trigger the bookmarklet.
(To install, you'll have to manually make a new bookmark in your bookmarks bar with the URL as the code above.)