[Feature Request]: double-sign detection

### Implementation ideas

Currently during sync, blocks are processed in First Come, First Served manner. After first block at given height is processed, other at this height are completely ignored. In case of double-sign of blocks at given height no action is taken (as latter block is ignored).

This is a missed opportunity to detect double-sign of a block.

Currently sequencer is centralized, so double-sign attack should have very significant consequences and probably should be resolved via human consensus.

首先定位同步區塊處理的進入點，以及對處於相同高度的區塊進行先到先得處理的位置。該 issue 未指定檔案或測試，也未定義偵測回應、人工共識流程和驗收標準；請在實作之前釐清這些內容。

由索引模型根據 Issue 內容生成。