Integrity constraint violation: 1062 Duplicate entry

Every now and then I'm getting messages from the amazon module in the watchdog like this:

Failed to insert item into amazon table: Message = SQLSTATE[23000]: Integrity constraint violation: 1062 Duplicate entry '3954515695' for key 'PRIMARY', query= INSERT INTO {amazon_book} (asin, isbn, ean, numberofpages, publicationdate) VALUES (:db_insert_placeholder_0, :db_insert_placeholder_1, :db_insert_placeholder_2, :db_insert_placeholder_3, :db_insert_placeholder_4) fields=

Array
(
    [asin] => 3954515695
    [isbn] => 3954515695
    [ean] => 9783954515691
    [numberofpages] => 384
    [publicationdate] => 2015-06-18
)

or

Failed to insert item into amazon_item_image table: Message = SQLSTATE[23000]: Integrity constraint violation: 1062 Duplicate entry '3954517108-smallimage' for key 'PRIMARY', query= INSERT INTO {amazon_item_image} (asin, size, height, width, url) VALUES (:db_insert_placeholder_0, :db_insert_placeholder_1, :db_insert_placeholder_2, :db_insert_placeholder_3, :db_insert_placeholder_4) fields=

Array
(
    [asin] => 3954517108
    [size] => smallimage
    [height] => 75
    [width] => 49
    [url] => https://images-eu.ssl-images-amazon.com/images/I/5156d9ziVFL._SL75_.jpg
)

Is there anybody, who can explain what this means and how I can avoid this?