Background: THV implantation within failed surgical valves is well established. However, the implications of THV implantation within failed THVs are poorly understood. Aims: This study aimed to assess the impact of different transcatheter heart valve (THV) designs and implant positioning strategies on hydrodynamic performance after redo transcatheter aortic valve implantation (TAVI). Methods: THVs of varying design (SAPIEN 3, Evolut PRO, ACURATE neo, ALLEGRA, and Portico) and size were implanted inside SAPIEN XT and Evolut R THVs. Hydrodynamic function as per International Organization for Standardization (ISO) specifications was evaluated using a pulse duplicator, and multimodality imaging was performed. Results: The majority of tested THV-in-THV combinations resulted in stable anchoring of the new implant. However, some combinations resulted in unstable anchoring with the potential for dislodgement or embolisation. Hydrodynamic function was favourable for all tested THV designs implanted in the intraannular SAPIEN XT THV. SAPIEN 3 implantation within an Evolut THV with supra-annular leaflets must be appropriately sized to ensure adequate anchoring. Avoidance of an intra-annular deployment mitigated leaflet overhang of the Evolut leaflets and optimised hydrodynamic performance. Conclusions: This study demonstrates that most THV designs and implantation strategies can result in favourable hydrodynamic performance following redo TAVI. Whether the leaflets of a failed THV are intraor supra-annular may have important implications for the positioning of a redo-THV implant. Certain THV designs or implantation positions may be more desirable when performing redo TAVI.