Are you using an English keyboard and/or English local aka Pd is in English? There seems to be  weird bug that causes this on *some* systems and switching Pd to english fixes it. I'm not sure what's going on, but it's related to the TK 8.5 cocoa bindings backend which is also causing the issues [key].

On Mar 22, 2018, at 11:34 AM, pd-list-request@lists.iem.at wrote:

Date: Thu, 22 Mar 2018 11:18:34 +0100
From: Jack <jack@rybn.org>
To: pd-list@lists.iem.at
Subject: [PD] Creating a GUI object on Pd 0.48.1 with keyboard
shortcut create two objects on MacOSX
Message-ID: <1fd3cfb5-f043-f751-4c0d-5cb3542c2513@rybn.org>
Content-Type: text/plain; charset=windows-1252

Hello,

Everything is in the title (Pd 0.48.1, MacOSX) : during a workshop some
people with this configuration trying to create a GUI object with
keyboard shortcut, create two times this object. (I don't know exactly
the version of MacOSX).
Is it a known bug ?
++

Jack

--------
Dan Wilcox
@danomatika
danomatika.com
robotcowboy.com