HPLC unknown peak in solvent front first sample injection

i'am having problems with an unknown peak in the solvent front of my assay.
After the pre-run injections, the first sample injection has an unknown peak (see photo). Also, the solvent front is always higher in the first sample injection and goes down in consecutive injections.
Does anyone know what causes the solvent front to be so high and why there appears a unknown peak in only the first sample injection. 
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