1. If the signature is written by yourself, you can go directly to the judicial department for technical appraisal of the handwriting;

2. If the content is written by the other party, and after you sign it, the supplementary content and the original content obviously belong to two parts, and they are written in two strokes, so don't worry, because legally, the signature can only be one-to-one, and the two strokes must have two signatures to take effect at the same time;

3. If the content is written by the other party, and after you sign it, there is no flaw in the task between the supplementary content and the original content, then you can only find relevant witnesses and evidence to prove that this evidence is suspected of being supplemented later.