When summoning a mob with multiple {"text": ... } attributes in its custom name, any text beyond the first will display twice.

Command tested was: 

/summon minecraft:creeper ~ ~ ~ {CustomName:'[{"text":"Test"},{"text":" Custom"},{"text":" Names"}]'}

In previous versions, it would summon a creeper with the name "Test Custom Names", but in 20w17a, it summons one named "Test Custom Names Custom Names".