Single tx included in two different blocks https://www.blockchain.com/btc/tx/a7bacce00e5df85d3547a3f83ac10c65eec30f715c1d77d8d4a7bc17fd470965

How is it possible that a single tx is included in two different blocks?

Isn't it considered double spending?
Occasionally, minor alternate chains emerge if multiple blocks are found for a given blockheight. Usually, these alternate chains only last for a single blocked, and are quickly dropped once another block has been found, allowing one chain to become longer (and thus have more work).

More rarely, these chains might last for a couple of blocks.

Block 525890 for this tx is not part of the main Bitcoin chain, and thus the tx was unspent when block 525891 was processed.

525889 ------ 525890 ------ 525891*
      \------ 525890*


The blocks marked with * contain the transaction in question here. As you can see, they occur on separate, parallel chains. Of these, only the chain extending 525891 survived, resulting in the version of block 525890 containing the transaction being dropped.