[One (M7) Q&A, Help & Troubleshooting] majmoz:
After the first

fastboot flash zip firmware.zip

you will something like this:

sending 'zip' (71868 KB)...
OKAY [4.936s]
writing 'zip'...
(bootloader) zip header checking...
(bootloader) zip info parsing...
(bootloader) checking model ID...
(bootloader) checking custom ID...
(bootloader) start image[hboot] unzipping for pre-update check...
(bootloader) start image[hboot] flushing...
(bootloader) [RUU]WP,hboot,0
(bootloader) [RUU]WP,hboot,99
(bootloader) [RUU]WP,hboot,100
(bootloader) ...... Successful
FAILED (remote: 90 hboot pre-update! [COLOR="Blue"]please flush image again immediately[/COLOR])
finished. total time: 6.410s

You noticed it says FAILED but then please flush image again immediately, this is your clue to flash the firmware again using the same command as the first time:

fastboot flash zip firmware.zip

Then you should see something like this:

sending 'zip' (71868 KB)...
OKAY [4.884s]
writing 'zip'...
(bootloader) zip header checking...
(bootloader) zip info parsing...
(bootloader) checking model ID...
(bootloader) checking custom ID...
(bootloader) start image[adsp] unzipping & flushing...
(bootloader) [RUU]WP,adsp,0
(bootloader) [RUU]WP,adsp,100
(bootloader) ...... Successful
(bootloader) start image[cir] unzipping & flushing...
(bootloader) ...... Successful
(bootloader) start image[rpm] unzipping & flushing...
(bootloader) [RUU]WP,rpm,0
(bootloader) [RUU]WP,rpm,100
(bootloader) ...... Successful
(bootloader) start image[sbl1-1] unzipping & flushing...
(bootloader) signature checking...
(bootloader) verified fail
(bootloader) ..... Bypassed
(bootloader) start image[sbl1-2] unzipping & flushing...
(bootloader) signature checking...
(bootloader) verified fail
(bootloader) ..... Bypassed
(bootloader) start image[sbl1-3] unzipping & flushing...
(bootloader) signature checking...
(bootloader) [RUU]WP,sbl1-3,0
(bootloader) [RUU]WP,sbl1-3,100
(bootloader) ...... Successful
(bootloader) start image[sbl2] unzipping & flushing...
(bootloader) [RUU]WP,sbl2,0
(bootloader) [RUU]WP,sbl2,100
(bootloader) ...... Successful
(bootloader) start image[sbl3] unzipping & flushing...
(bootloader) [RUU]WP,sbl3,0
(bootloader) [RUU]WP,sbl3,100
(bootloader) ...... Successful
(bootloader) start image[tp] unzipping & flushing...
(bootloader) ...... Successful
(bootloader) start image[tz] unzipping & flushing...
(bootloader) [RUU]WP,tz,0
(bootloader) [RUU]WP,tz,100
(bootloader) ...... Successful
(bootloader) start image[radio] unzipping & flushing...
(bootloader) [RUU]WP,radio,0
(bootloader) [RUU]WP,radio,26
(bootloader) [RUU]WP,radio,53
(bootloader) [RUU]WP,radio,79
(bootloader) [RUU]WP,radio,100
(bootloader) ...... Successful
OKAY [ 39.601s]
finished. total time: 44.485s

Now you want to reboot the phone and then flash the next one.

One thing I failed to mention was that in order to successfully flash the firmware you will need to first "Relocked" your bootloader. You can use this command:

fastboot oem lock

After the firmware are successfully flash you will need to "Unlocked" the bootloader, if you have your old Unlock_code.bin you can try it. If not then you will have to use the HTCDev method. If you have any questions don't hesitate to ask.