a_1 rt
a_1 st_2
a_1 st_3
a_2 bt_2
a_2 st_er
b_2 st_2
b_2 st_32
S_1 rt_8
S_1 rt_64

a_1 rt st_2  st_3
a_2 bt_2 st_er
b_2 st_2 st_32
S_1 rt_8 rt_64

awk '
    { stuff[$1] = stuff[$1] $2 " " }
    END {
        for( s in stuff )
            print s, stuff[s];
    }' input-file |sort

awk '
    {
        if( !seen[$1]++ )
            order[++oidx] = $1;
        stuff[$1] = stuff[$1] $2 " "
    }
    END {
        for( i = 1; i <= oidx; i++ )
            print order[i], stuff[order[i]]
    }
' input-file