Whether you must remove old thinset before installing new tile depends entirely on the condition, flatness, and structural adhesion of the existing layer. If the old thinset is firmly bonded, completely flat, structurally sound, and thoroughly cleaned of all loose debris, dust, and grease, you can sometimes install a new layer over it after applying an appropriate primer. However, if the old mortar is crumbling, thick, uneven, or contaminated with old mastic adhesive, it must be completely scraped, ground, or scarified away. Laying new tile over a compromised, uneven, or thick buildup of old thinset severely compromises the bonding plane, leading to catastrophic tile failure, cracking, and detachment.
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