JEP 401 says:

(Possibly) The constructor cannot make use of this except to set the fields in the constructor body, or perhaps after all fields are definitely assigned.

// --

The code that implements this seems to have a bug wherein duplicate errors are emitted. Consider:

public primitive class X implements Cloneable {
    int x;
    int y;
    X() {
        foo(this);
    }
    void foo(X x) {
    }
}

Compiling this results in:

X.java:5: error: primitive class instance should not be passed around before being fully initialized
        foo(this);
            ^
X.java:5: error: primitive class instance should not be passed around before being fully initialized
        foo(this);
           ^
X.java:6: error: variable x might not have been initialized
    }

The first two errors are duplicates of each other. They seem to differ in DiagnosticPosition by 1 so both show up. But the concerned AST node is the same - it is to be examined why two messages appear.