Elasticsearch conflicts with existing mapping in other types
An index consists of multiple types (in version 6 this is no longer possible mainly due to this reason). The problem with different types is that they cannot store the same field name with a different mapping. This has to do with the way it is stored in Lucene.
Could it be you are inserting documents in two different types? Maybe by accident (Typo in the type while ingesting documents for instance). Then it might try to create a different field type by dynamic mapping say a string. This would cause the exception that you mention.