I need to convert my master data to county level. For that in my master data I have statefip and city name. In my using data I have city , statefip , county_fip and county name.

The issue arises, since plenty of city in different states have multiple county code/county name as I'm showing a part of duplicates ( from my using data ) data with same city and state but different county name /couty_fip code.

Code:
list city state_id county_name, sepby(city state_id) noobs

  +---------------------------------------+
  |        city   state_id    county_name |
  |---------------------------------------|
  |      Midway         FL        Gadsden |
  |      Midway         FL     Santa Rosa |
  |      Midway         FL       Seminole |
  |---------------------------------------|
  |    Woodbury         NY         Nassau |
  |    Woodbury         NY         Orange |
  |---------------------------------------|
  |     Oakwood         OH       Cuyahoga |
  |     Oakwood         OH     Montgomery |
  |     Oakwood         OH       Paulding |
  |---------------------------------------|
  |    Franklin         PA        Cambria |
  |    Franklin         PA        Venango |
  |---------------------------------------|
  |  Georgetown         PA         Beaver |
  |  Georgetown         PA      Lancaster |
  |  Georgetown         PA        Luzerne |
  |---------------------------------------|
This is how my master data looks like :

dataex id city statefip if city=="Georgetown" & statefip=="PA"

----------------------- copy starting from the next line -----------------------

Code:
* Example generated by -dataex-. For more info, type help dataex
clear
input str14 id str43 city str3 statefip
"11311793" "Georgetown" "PA"
"12054868" "Georgetown" "PA"
"13761470" "Georgetown" "PA"
"63145796" "Georgetown" "PA"
"63058645" "Georgetown" "PA"
"11415903" "Georgetown" "PA"
"62891646" "Georgetown" "PA"
"10933939" "Georgetown" "PA"
"17385483" "Georgetown" "PA"
"10925795" "Georgetown" "PA"
end
This is how my using data looks like

dataex city statefip county_fips county_name if city== "Georgetown" & statefip=="PA"

----------------------- copy starting from the next line -----------------------
Code:
* Example generated by -dataex-. For more info, type help dataex
clear
input str35 city str2 statefip long county_fips str21 county_name
"Georgetown" "PA" 42007 "Beaver"   
"Georgetown" "PA" 42071 "Lancaster"
"Georgetown" "PA" 42079 "Luzerne"  
end

When I'm using m:1

Code:
 
merge m:1 city statefip using "using.dta", keep(master match) nogen
variables city statefip do not uniquely identify observations in the using data
r(459);

end of do-file

r(459);
Whenever I'm using 1:m the following error message shows up

Code:
merge 1:m city statefip using "city_countyfipshort.dta", keep(master match) nogen
variables city statefip do not uniquely identify observations in the master data
r(459);

end of do-file

r(459);
What direction should I go to given this particular problem ?