Highlight cell if text is a duplicate within the last 31 days

I will try to keep this brief. I trade stock market options in my retirement account and I created a simple spreadsheet to help track my trades. What I'm trying to accomplish is to have the cell in column F highlight yellow whenever I enter a symbol for a new trade that I'm about to open. If I have previously traded said symbol within the last 31 days then I would like for the cell to highlight to yellow, as a reminder to not trade that symbol and choose a different symbol to trade. Or any similar method if its seems simpler, I am open to ideas. I just need a way to prevent trading the same symbol within the same month, specifically the same symbol that I previously lost on in the last 31 days...but I will get into that later. I have searched the web for a week and I've tried a few ideas but nothing seems to work. I'm using google sheets for this spreadsheet.

After all the searching I've done, this is the conditional formula for column F that I found searching this forum that I thought would work...

=and(countif(feb!f$9:f9,f9)>1,today()-feb!ac9<31

...but its not working.

I have 13 pages on 1 spreadsheet from "Jan 2021" to "Dec 2021". We are in March 2021 right now so the formula above I had entered in conditional format for cell March!F9. All pages are the same copy. The symbols column range from F9:F1000, The "exit date" column range from AC9:AC1000. I do also have a second exit date that I'd like to incorporate, but I will get into that later as well.

I figured Id start here first to keep things simple and organized, and after its working, I'll throw some more curve balls into the mix.

Seriously, thank you very much!