WITH SINGLE INSERT: bulk insert id (not primary),meta_key,meta_value into table2 from table1 where each `id`+`meta_key` pair do not exist in table2

I am trying to bulk insert data into wp_postmeta from the results of select from wp_posts:

1. within one INSERT query (I do not want to process data in PHP, only send a request to MySQL)
2. not duplicating rows if they exist.

The primary key in wp_postmeta can't be used to check for duplicate keys (it's an autoincrement key and I don't have it in the select data)

So my code is

INSERT INTO `wp_postmeta`
        (`post_id`,
         `meta_key`,
         `meta_value`
        )
        SELECT
              `ID`,
              'my_id',
              `post_excerpt`,
        FROM `wp_posts` WHERE post_status = 'myprefix_pending'

this code works, but duplicates rows on every execution

I have modified it the following way:

INSERT INTO `wp_postmeta`
        (`post_id`,
         `meta_key`,
         `meta_value`
        )
        SELECT
              `ID`,
              'my_id',
              `post_excerpt`,
        FROM `wp_posts` WHERE post_status = 'myprefix_pending' AND NOT EXISTS(SELECT meta_value FROM `wp_postmeta` WHERE ID=`ID` AND meta_key='1C_id')

But this part doesn't work:

AND NOT EXISTS(SELECT meta_value FROM `wp_postmeta` WHERE ID=`ID` AND meta_key='1C_id')

How do I do it the right way?

I've read SQL Multiple WHERE IN but don't understant if it answers my question