Append a and b (etc) to the year to differentiate citations and references.
EndNote knows how to do this.
Add author initials to differentiate.
EndNote knows how to do this, but it does it for all authors, not just primary authors - this is a reasonable extrapolation and interpretation of the APA rules (which do not specify disambiguation in this case).
Add authors until citations are different:
EndNote knows how to do this.
A surprisingly common experience: Many editors and supervisors are either not familiar with the above rules, or regard them as too complex, so they would prefer to see all in-text citations as (Smith, 2015a) and (Smith, 2015b) or (Jones et al., 2015a) and (Jones et al., 2015b) even if these are different primary authors or have other different co-authors.
Note: this will change the main EndNote program style settings on a specific computer, it does not change within your EndNote library, so you cannot take your EndNote library and Word file to a different computer and retain the changed style.
1. In EndNote, ensure you have selected style you wish to modify
2. Tools menu > Output Styles > Edit "<name of style>" (e.g. Tools > Output Styles > Edit "APA 7th")
3. Under Citations , select Ambiguous Citations
4. Change to the Author Name section, still under Citations.
5. Use File menu > Save As to save your modified style.
6. Change the name to indicate your changes to the style
7. Select your modified style for use in EndNote and Word