BUG #16088: Multiple foreigh keys created on "ALTER TABLE" command

The following bug has been logged on the website:

Bug reference:      16088
Logged by:          Jess Sturgeon
Email address:      jsturgeon@novetta.com
PostgreSQL version: 12.0
Operating system:   RHEL 7.7
Description:

If you run this command multiple times:

ALTER TABLE myschema.mytable ADD IF NOT EXISTS mycolumn uuid REFERENCES
myschema.mytable2(userid);

It adds the column once, but adds a duplicate FOREIGN KEY each time.  I had
over 59 keys while debugging my schema definition :)

This happens in PG11.5 and PG12 for me, both on RHEL 7.7