A short awk program will achieve this for you

awk -F': ' '
    # Every line
    {
        # Append a comma if we have previous items
        if (h[$1] > "") { h[$1] = h[$1] ", " };

        # Append the item and increment the count
        h[$1] = h[$1] $2;
        i[$1]++
    }

    # Finally
    END {
        # Iterate across all the keys we have found
        for (k in h) {
            if (i[k] > 1) { p = "[%s]" } else { p = "%s" };
            printf "%s: " p "\n", k, h[k]
        }
    }
' data.txt

Output

hmz_age: ['21', '41']
tom_age: '31'
fd_year_anne: ['1987', '1982']
school: ['anne', 'svp']
name: ['tom', 'hmz', 'toli']
status_hmz_mar: 'no'
status_tom_mar: 'yes'

A short awk program will achieve this for you

awk -F': ' '
    # Every line of input; fields split at colon+space
    {
        # Append a comma if we have previous items
        if (h[$1] > "") { h[$1] = h[$1] ", " };

        # Append the item and increment the count
        h[$1] = h[$1] $2;
        i[$1]++
    }

    # Finally
    END {
        # Iterate across all the keys we have found
        for (k in h) {
            if (i[k] > 1) { p = "[%s]" } else { p = "%s" };
            printf "%s: " p "\n", k, h[k]
        }
    }
' data.txt

Output

hmz_age: ['21', '41']
tom_age: '31'
fd_year_anne: ['1987', '1982']
school: ['anne', 'svp']
name: ['tom', 'hmz', 'toli']
status_hmz_mar: 'no'
status_tom_mar: 'yes'

A short awk program will achieve this for you

awk -F': ' '
    {
        if (h[$1] > "") { h[$1] = h[$1] ", " };
        h[$1] = h[$1] $2;
        i[$1]++
    }

    END {
        for (k in h) {
            if (i[k] > 1) { p = "[%s]" } else { p = "%s" };
            printf "%s: " p "\n", k, h[k]
        }
    }
' data.txt

Output

hmz_age: ['21', '41']
tom_age: '31'
fd_year_anne: ['1987', '1982']
school: ['anne', 'svp']
name: ['tom', 'hmz', 'toli']
status_hmz_mar: 'no'
status_tom_mar: 'yes'

A short awk program will achieve this for you

awk -F': ' '
    # Every line
    {
        # Append a comma if we have previous items
        if (h[$1] > "") { h[$1] = h[$1] ", " }; 

        # Append the item and increment the count
        h[$1] = h[$1] $2;
        i[$1]++
    } 

    # Finally
    END {
        # Iterate across all the keys we have found
        for (k in h) {
            if (i[k] > 1) { p = "[%s]" } else { p = "%s" };
            printf "%s: " p "\n", k, h[k]
        }
    }
' data.txt

Output

hmz_age: ['21', '41']
tom_age: '31'
fd_year_anne: ['1987', '1982']
school: ['anne', 'svp']
name: ['tom', 'hmz', 'toli']
status_hmz_mar: 'no'
status_tom_mar: 'yes'