Craft doesn't check for unique URI correctly when saving page "as a new entry" if the URI is generated using a field

[coryzibell]

Description

I have a section where the URI structure is a field:

When attempting change that field and save an entry as a new entry, Craft throws an error that it cannot generate a unique URI, even though that field has been changed.

Steps to reproduce

1. Make a section where the URI is a reference to a field
2. Make an entry in that section
3. Open the entry, change the field that is used for the URI, and change the slug, attempt to "save the page as a new entry"
4. Craft will report it cannot generate a unique URI

Additional info

• Craft version: 3.0.25
• PHP version: 7.1.20
• Database driver & version: MySQL 5.5.5
• Plugins & versions:
  Architect 2.2.10
  Calendar Links 1.0.0
  Child Me! 1.0.4
  Cookies 1.1.10
  CP Field Inspect 1.0.4
  Embedded Assets 1.0.2
  Environment Label 3.1.4
  Feed Me 3.0.0-beta.21
  Fetch 1.2.0
  Guide 1.3.2
  Inlin 2.0.0
  Inventory 2.0.2
  Position Fieldtype 1.0.13
  Redactor 2.1.6
  SEO 3.4.4
  Spoon 3.2.3
  Super Table dev-craft-3
  Twig Perversion 2.0.5