Fields duplicated on multiple executions

Problem/Motivation

Duplicate fields created when migration runs a second time, e.g.:

[error] 'field_storage_config' entity with ID 'block_content.field_button' already exists. (/var/www/html/web/core/lib/Drupal/Core/Entity/EntityStorageBase.php:485)

Steps to reproduce

Run a migration (migrate:import).
Wait for it to fail on something.
Run the migration again (migrate:import).

Proposed resolution

Fix it so that fields for ECK items are not generated more than once.

Remaining tasks

Fix the bug (-:

User interface changes

n/a

API changes

n/a

Data model changes

n/a