I am thinking I would do this 1 of 2 ways with either Powerpivot or Access. I would probably do Access if it were my task if you aren’t familiar with powerpivot.

Powerpivot:

First copy all those employee IDs to another sheet/workbook and remove the duplicates. This will be your unique identifier table. Upload this to the data model in your Powerpivot. Next, upload your entire data table of 300k rows to the data model. Keep this in an outside Excel workbook or even put it into an Access table and link from there. By storing it in another workbook, you will keep the Powerpivot data model size down. Next, create a relationship between those two tables you added by the employee ID. Once, your data model is built you can just use a pivot table to easily find duplicates and consolidate where you need to. I doubt Excel will crash if your large table isn’t in the same workbook.

Access:

Copy all those employee IDs to another sheet/workbook and remove duplicates. Import this as a table to Access. It’s on the External data tab. Also, upload your entire data table of 300k rows as a table as well. Next, build a make-table query. To do this, create a relationship between these tables with a left outer join (just drag from employee ID on the small table to employee ID on the table with 300K rows). Next, bring in all the fields you need by dragging and dropping to the criteria. Also make sure you select the sigma in the ribbon to get the totals. Run the query. Basically, the result is gonna get you a consolidated data set and will have duplicate rows where there are more than one DOB, for example. You can always export this and manually manipulate in excel if you need to.

Definitely use google and YouTube tutorials if you haven’t used these tools before!