You could use a HashSet to store unique PVector objects (must differ in either x, y or z to be unique) but you can't use a collection that is naturally ordered/sorted e.g. TreeSet.

The example sketch proves that the HashSet only stores unique PVector object, If you change  HashSet to TreeSet in line 3 then the sketch fails at line 15 when it tries to add a second PVector object.

1. import java.util.*;
2. PVector p0,p1,p2;
3. HashSet<PVector> aset = new HashSet<PVector>();
4. 
   
5. void setup(){
6.   p0 = new PVector(1,2,3);
7.   p1 = new PVector(1,2,3);
8.   p2 = p1;
9.  
10.   println( (p0.equals(p1)) + " should be TRUE");
11.   println( (p1.equals(p2)) + " should be TRUE");
12.  
13.   aset.add(p0);
14.   println(aset.size() + "  should be 1");
15.   aset.add(p1);
16.   println(aset.size() + "  should be 1");
17.   aset.add(p2);
18.   println(aset.size() + "  should be 1");
19. }