So you only have the first column? And would like to generate the second? I think a slightly different awk command could work
awk '{if (last==$1) {flag=0} else {last=$1; flag=1}; print $0,flag}' file.txt
Basically you just check if the first field matches the last one you've seen. Since it's sorted, you don't have to keep track of everything you've seen, only the last one to know if the value is different.