feeds_item this field cannot hold more than 1 values.

Using multiple feeds to update the same content is a supported feature. The feeds_item field used to be a single value field, but it was changed to a multivalue field a few releases ago. Existing feeds_item fields were updated by running feeds_post_update_ensure_feeds_item_storage_config_cardinality_is_unlimited(). This function should have run automatically when running database updates (drush updatedb or update.php). Perhaps you accidentally reverted the config of some feeds_item fields after that update ran?

You should be able to fix this issue by manually editing the field.storage.node.feeds_item.yml config file and set "cardinality" to -1. Or if you importing entities of an other type, change it into the field.storage.[entity type].feeds_item.yml config file. After changing the config file, do a config import.

Example of field.storage.node.feeds_item.yml on my custom project:

uuid: 1d2038b7-1b62-4ab6-ad4b-c0b6f1734b61
langcode: nl
status: true
dependencies:
  module:
    - feeds
    - node
id: node.feeds_item
field_name: feeds_item
entity_type: node
type: feeds_item
settings:
  target_type: feeds_feed
module: feeds
locked: false
cardinality: -1
translatable: false
indexes: {  }
persist_with_no_fields: false
custom_storage: false

Great thanks for the info, I must have overwritten the config somehow.

Automatically closed - issue fixed for 2 weeks with no activity.