Ruby, 105(削除) 105 (削除ここまで) 129 bytes
+24 bytes because I missed part of the spec
->a{a.map{|e|i=a|e|l=e[r=/\S+$/];i=a.count{|n|n[/\S+/]==e[/\S+$/]]==l};e+"#{' definitely'if i>1i>a.count{|n|n[r]==l}} has two first names"if i>0}-[p]}
Ruby, 105 bytes
->a{a.map{|e|i=a.count{|n|n[/\S+/]==e[/\S+$/]};e+"#{' definitely'if i>1} has two first names"if i>0}-[p]}
Ruby, (削除) 105 (削除ここまで) 129 bytes
+24 bytes because I missed part of the spec
->a{a.map{|e|l=e[r=/\S+$/];i=a.count{|n|n[/\S+/]==l};e+"#{' definitely'if i>a.count{|n|n[r]==l}} has two first names"if i>0}-[p]}