On Fri, 09 Oct 2020 10:16:47 +0100, NY wrote:

There aren't many locks where you can insert a key from the outside if
there is a key on the inside. The standard locks fitted to patio doors
etc will accept the second key - it feels as if it has gone all the way
in - but it will not turn if there is a key on the other side.

Change the cylinder for an Anti Lockout one.



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