How filter records with same field value

It depends on how you want the result to be presented. If you are happy with a field containing the list of unique treatments, you can put this in a formula field:

concat(unique(Treatments.Name))

or, if you want a multiline list:

join(unique(Treatments.Name), "
")

Also, nothing forbids to add a sort() function if you want the treatment names to be ordered.

If what you need is a tabular view, I have no idea on how to do it in your two-tables database. It would be doable in a database with three tables, organizing a m:n relationship between Patients, and a table containing the list of all possible treatments. Such a database structure, admittedly more complex, would probably  have some other benefits.

Before considering a redesign of your database, there are some things you could try to see if they give acceptable results.

1- create a list view in the Treatments table ; be sure to include at least a column with the treatment 'name', and a column with the patient name, by reference to the Patients table ; click on the header of the patient name column, and set the filter field to the name of a patient ; click on the header of the treatment name, and select the 'Grouped' option.

2- in the Patients table, the default form view should contain a tabular view of the linked treatments ; you could click on the header of the treatment name colmun, and select the 'Grouped' option.