Duplicate item warnings

When I update any of my rules files I am getting duplicate field warnings for items I have defined in an items file. Does anyone know why this is happening, am I doing something wrong or is this an internal issue?

2017-07-15 20:44:54.288 [WARN ] [.rule.jvmmodel.RulesJvmModelInferrer] - Duplicate field: 'Light_MF_OF'. Ignoring 'org.eclipse.smarthome.core.library.items.SwitchItem'.
2017-07-15 20:44:54.288 [WARN ] [.rule.jvmmodel.RulesJvmModelInferrer] - Duplicate field: 'Light_MF_DK'. Ignoring 'org.eclipse.smarthome.core.library.items.SwitchItem'.
2017-07-15 20:44:54.288 [WARN ] [.rule.jvmmodel.RulesJvmModelInferrer] - Duplicate field: 'Light_MF_NK'. Ignoring 'org.eclipse.smarthome.core.library.items.SwitchItem'.
2017-07-15 20:44:54.288 [WARN ] [.rule.jvmmodel.RulesJvmModelInferrer] - Duplicate field: 'Light_MF_FD'. Ignoring 'org.eclipse.smarthome.core.library.items.SwitchItem'.
2017-07-15 20:44:54.288 [WARN ] [.rule.jvmmodel.RulesJvmModelInferrer] - Duplicate field: 'Light_MF_BW'. Ignoring 'org.eclipse.smarthome.core.library.items.SwitchItem'.
2017-07-15 20:44:54.288 [WARN ] [.rule.jvmmodel.RulesJvmModelInferrer] - Duplicate field: 'Light_MF_BW2'. Ignoring 'org.eclipse.smarthome.core.library.items.SwitchItem'.
2017-07-15 20:44:54.288 [WARN ] [.rule.jvmmodel.RulesJvmModelInferrer] - Duplicate field: 'Light_MF_KT'. Ignoring 'org.eclipse.smarthome.core.library.items.SwitchItem'.
2017-07-15 20:44:54.288 [WARN ] [.rule.jvmmodel.RulesJvmModelInferrer] - Duplicate field: 'Light_MF_SK'. Ignoring 'org.eclipse.smarthome.core.library.items.SwitchItem'.
2017-07-15 20:44:54.288 [WARN ] [.rule.jvmmodel.RulesJvmModelInferrer] - Duplicate field: 'Light_LF_LT'. Ignoring 'org.eclipse.smarthome.core.library.items.SwitchItem'.
2017-07-15 20:44:54.288 [WARN ] [.rule.jvmmodel.RulesJvmModelInferrer] - Duplicate field: 'Light_2F_VLT'. Ignoring 'org.eclipse.smarthome.core.library.items.DimmerItem'.
2017-07-15 20:44:54.288 [WARN ] [.rule.jvmmodel.RulesJvmModelInferrer] - Duplicate field: 'Light_MF_DNG'. Ignoring 'org.eclipse.smarthome.core.library.items.DimmerItem'.
2017-07-15 20:44:54.288 [WARN ] [.rule.jvmmodel.RulesJvmModelInferrer] - Duplicate field: 'Motion_MF_BW'. Ignoring 'org.eclipse.smarthome.core.library.items.SwitchItem'.
2017-07-15 20:44:54.300 [INFO ] [el.core.internal.ModelRepositoryImpl] - Refreshing model 'sunset.rules'
2017-07-15 20:44:54.302 [WARN ] [.rule.jvmmodel.RulesJvmModelInferrer] - Duplicate field: 'Light_MF_OF'. Ignoring 'org.eclipse.smarthome.core.library.items.SwitchItem'.
2017-07-15 20:44:54.302 [WARN ] [.rule.jvmmodel.RulesJvmModelInferrer] - Duplicate field: 'Light_MF_DK'. Ignoring 'org.eclipse.smarthome.core.library.items.SwitchItem'.
2017-07-15 20:44:54.302 [WARN ] [.rule.jvmmodel.RulesJvmModelInferrer] - Duplicate field: 'Light_MF_NK'. Ignoring 'org.eclipse.smarthome.core.library.items.SwitchItem'.
2017-07-15 20:44:54.302 [WARN ] [.rule.jvmmodel.RulesJvmModelInferrer] - Duplicate field: 'Light_MF_FD'. Ignoring 'org.eclipse.smarthome.core.library.items.SwitchItem'.
2017-07-15 20:44:54.302 [WARN ] [.rule.jvmmodel.RulesJvmModelInferrer] - Duplicate field: 'Light_MF_BW'. Ignoring 'org.eclipse.smarthome.core.library.items.SwitchItem'.
2017-07-15 20:44:54.302 [WARN ] [.rule.jvmmodel.RulesJvmModelInferrer] - Duplicate field: 'Light_MF_BW2'. Ignoring 'org.eclipse.smarthome.core.library.items.SwitchItem'.
2017-07-15 20:44:54.302 [WARN ] [.rule.jvmmodel.RulesJvmModelInferrer] - Duplicate field: 'Light_MF_KT'. Ignoring 'org.eclipse.smarthome.core.library.items.SwitchItem'.
2017-07-15 20:44:54.302 [WARN ] [.rule.jvmmodel.RulesJvmModelInferrer] - Duplicate field: 'Light_MF_SK'. Ignoring 'org.eclipse.smarthome.core.library.items.SwitchItem'.
2017-07-15 20:44:54.302 [WARN ] [.rule.jvmmodel.RulesJvmModelInferrer] - Duplicate field: 'Light_LF_LT'. Ignoring 'org.eclipse.smarthome.core.library.items.SwitchItem'.
2017-07-15 20:44:54.302 [WARN ] [.rule.jvmmodel.RulesJvmModelInferrer] - Duplicate field: 'Light_2F_VLT'. Ignoring 'org.eclipse.smarthome.core.library.items.DimmerItem'.
2017-07-15 20:44:54.303 [WARN ] [.rule.jvmmodel.RulesJvmModelInferrer] - Duplicate field: 'Light_MF_DNG'. Ignoring 'org.eclipse.smarthome.core.library.items.DimmerItem'.
2017-07-15 20:44:54.303 [WARN ] [.rule.jvmmodel.RulesJvmModelInferrer] - Duplicate field: 'Motion_MF_BW'. Ignoring 'org.eclipse.smarthome.core.library.items.SwitchItem'.

I’ve had this occasionally too with 2.1, and there are definitely no duplications in the items files.