Hi guyfromri, and welcome to the forum!
As you have v10, this'll be easier for you than it is for others.
First, in the Table window, define a custom sort order based first on the chart number, and then on the date field (descending).
Now define a new filter. Name it No Duplicates and instead of messing with the formula, go straight to the Advanced tab.
Click the "Select row according to ..." option, and check the duplicated rows box, with the first row only, as sorted. Check the box for the chart number on the right hand side of the dialog, and OK the filter and OK again for the list of filters.
If you have v7 "Pro" then I have a possible solution. If you don't then I'm afraid that I've come up empty.
First, add a calculated field named RecNo using the RecNo() function to your table.
Now sort the table by Location first (ascending), and then by LastVisit (descending).
Build a project export to export the entire table to a file (.xls, .xlsx, .mdb, etc.). Save the model and project.
Now create a new model/project, using that export as a datasource. Add to it an external lookup (to the same file[/B]) based on the Location fields. Import the RecNo field as LocationRecNo. You'll get a warning about not forming a unique key. That's OK. Click Yes and proceed.
Build a filter with the expression:
This should give you the unique list in the sequence that you want. Save the model and project when it's correct.
My method is a little simpler.
Open the Summary window (Window/Summary).
On the Summary Start Page, select New Summary - Complete.
On the General tab, name the summary and apply a filter if needed (if you don't specify one, it will default to what ever is in the table window). There's a checkbox if you want a total line.
On the Fields tab, select the fields that have the duplicate values and put them into the Key or Item fields.
Monarch will summarize the data based on what fields you specify.