My recommendation is to always test, test, test. Your default should be to test and use the title tag that gets you the combination of best rankings and highest click-through rate. And to find the optimal title tag, you should be testing one's that duplicate keywords too.
I'll give you an example but I'm going to change out the actual essential oil name used so my client remains anonymous.
On March 5th, a specific page was ranking on Google as follows:
- #19 for "catnip essential oil"
- #17 for "catnip oil"
- #7 for "where to buy catnip oil"
The title tag at the time on that page was: "Catnip Essential Oil | Website Name".
I changed the title tag to "Catnip Essential Oil - Where To Buy Catnip Oil | Website Name".
After the page was recrawled by Google the rankings moved up to better positions. Here are the results:
- "catnip essential oil" has been as high as #6 but is at #9 today.
- "catnip oil" has been as high as #8 but is at #10 today.
- "where to buy catnip oil" moved up to #1 and remains there today
- No change in click-through rate
As you can see the results are better even with duplicating the words "catnip" and "oil" in the title tag.
Someone suggested that I should have just changed the title tag to "Where to Buy Catnip Essential Oil | Website Name". That is a worthwhile test and maybe I'll do that still just to see what happens but for now I actually prefer having both "catnip essential oil" and "catnip oil" in my title tag.
Hopefully you can see from this real world result that you can have duplicate words in your title tag and that testing is more important than following so called "best practices".