Duplicate numbers counted

Well I am extremely new to anything more technical than inputting data into cells so I thought it best if I asked for help although I have tried unsuccessfully to figure it out

Basic problem I am a trying to sort multiple columns with different data for a returns inventory

Columns im using

Date created
Part number

The created date I want to use a rolling date of no longer than 6 months I was able to research and find out a formula in a separate column as

=(C4+183)>TODAY()

Which returns my dates as true / false I have then used conditional formatting to colour the sells red / green depending on the text in the cell.

What I would like to do now in a separate column / columns is indicate the duplicate numbers in the " part number " column

And then give me a count of how many times the same part number ( with in the 6 month period ) has been repeated and this has got me completely stumpped i have tried to search the forums to but was unable to find anything but that is probably me

Any help would be greatly appreciated

Thanks

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This part might lead to errors if say february is included in the 6 months
To be on the safe side try =date(year(c4),month(c4)+6,day(c4))>today()